…ax` example.

For ohler55#277

I've been experimenting with `Ractor`s since upgrading to Ruby 3.0 for ohler55#275
but quickly ran into `Ractor::UnsafeError` when trying to call `Ox::sax_parse` in anything except the main `Ractor`.

Per Ruby's `Ractor` C extension documentation (link below):
"By default, all C extensions are recognized as `Ractor`-unsafe. If C extension becomes `Ractor`-safe,
the extension should call `rb_ext_ractor_safe(true)` at the `Init_` function and all defined method marked as `Ractor`-safe.
`Ractor`-unsafe C-methods only been called from main-ractor. If non-main ractor calls it, then `Ractor::UnsafeError` is raised."

I don't like to open seemingly-large feature requests like this without making some attempt at it myself first,
and luckily it seems like `Ox::sax_parse` Just Works™ since I marked it `Ractor`-safe, even with the `class_cache`.
Confirming this safety, making any remaining changes to `Ox::Sax`, and expanding this to the non-`Sax` parts of `Ox`
are all unfortunately out of my depth as a n00b C coder, so I would appreciate if you could take this over if it interests you.
I am happy with just `Sax` support since I have no current need for marshalling,
but I imagine other `Ox` users wouldn't be satisfied if stratified.

In this commit:
- Enable `rb_ext_ractor_safe` preprocessor macro via `have_func` in `extconf.rb`.
- Mark `Init_Ox` and `ox_sax_parse` as `Ractor` -safe.
- Add a new `Ractor`-based `Ox::Sax` example exercising both parallel and serial `Ox::Sax` handler `Ractor`s
  to parse data from `shared-mime-info` XML files many users likely already have on their systems.

Official `Ractor` info:

- "Ractor: a proposal for a new concurrent abstraction without thread-safety issues": https://bugs.ruby-lang.org/issues/17100 (ruby/ruby#3365)
- Ruby's official `Ractor` documentation: https://docs.ruby-lang.org/en/master/doc/ractor_md.html
- "A way to mark C extensions as thread-safe, Ractor-safe, or unsafe": https://bugs.ruby-lang.org/issues/17307 (ruby/ruby#3824)
- Ruby's C Extension `Ractor` documention covering `rb_ext_ractor_safe`: https://docs.ruby-lang.org/en/master/doc/extension_rdoc.html#label-Appendix+F.+Ractor+support
- A `Ractor` C Extension from the creator of `Ractor` that might serve as a useful example: https://github.com/ko1/ractor-tvar

Blogs:

- "Ractors: Multi-Core Parallel Processing Comes to Ruby 3": https://www.ruby3.dev/ruby-3-fundamentals/2021/01/27/ractors-multi-core-parallel-processing-in-ruby-3/
- "Ruby Ractor Experiments: Safe async communication" :https://ivoanjo.me/blog/2021/02/14/ractor-experiments-safe-async/
- "Playing with Ruby Ractors": https://billy-ruffian.co.uk/playing-with-ruby-ractors/
- "How Fast are Ractors?": https://www.fastruby.io/blog/ruby/performance/how-fast-are-ractors.html (https://github.com/noahgibbs/ractor_basic_benchmarks/tree/main/benchmarks)

Before this change:

```
[okeeblow@emi#CHECKING-YOU-OUT] time ./bin/checking-you-out ~/224031-dot-jpg
Received /home/okeeblow/.local/share/mime/packages/user-extension-rsrc.xml
/home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:375:in `sax_parse': ractor unsafe method called from not main ractor (Ractor::UnsafeError)
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:375:in `block in open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:331:in `open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:331:in `open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:24:in `block (2 levels) in remember_me'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:19:in `loop'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:19:in `block in remember_me'
<internal:ractor>:583:in `send': The incoming-port is already closed (Ractor::ClosedError)
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:86:in `block in extended'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `each'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `each_with_object'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `extended'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/inner_spirit.rb:216:in `extend'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/inner_spirit.rb:216:in `<top (required)>'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out.rb:10:in `require_relative'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out.rb:10:in `<top (required)>'
        from ./bin/checking-you-out:3:in `require_relative'
        from ./bin/checking-you-out:3:in `<main>'
./bin/checking-you-out ~/224031-dot-jpg  0.12s user 0.05s system 100% cpu 0.168 total
```

My new `Ox::Sax` Ractor example script's usage:

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb
Please provide the path to a `shared-mime-info` XML package and some media-type query arguments (e.g. 'image/jpeg')
```

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml
Please provide some media-type query arguments (e.g. 'image/jpeg')
```

Finding all-extant types:

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml image/jpeg font/ttf application/xhtml+xml image/x-pict
Parallel Ractors
["Worker 0 gave us JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]",
 "Worker 1 gave us TrueType 字型 (font/ttf) [.ttf]",
 "Worker 2 gave us XHTML 網頁 (application/xhtml+xml) [.xhtml,.xht]",
 "Worker 3 gave us Macintosh Quickdraw/PICT 繪圖 (image/x-pict) [.pct,.pict,.pict1,.pict2]"]

Serial Ractor
"ONLY ONE OX gave us [#<CYO JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]>, #<CYO TrueType 字型 (font/ttf) [.ttf]>, #<CYO XHTML 網頁 (application/xhtml+xml) [.xhtml,.xht]>, #<CYO Macintosh Quickdraw/PICT 繪圖 (image/x-pict) [.pct,.pict,.pict1,.pict2]>]"
```

…and not finding invalid ones:

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml lol/rofl fart/butt image/jpeg
Parallel Ractors
["Worker 0 gave us nothing",
 "Worker 1 gave us nothing",
 "Worker 2 gave us JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]"]

Serial Ractor
"ONLY ONE OX gave us [nil, nil, #<CYO JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]>]"
```

Unit tests pass.

…ax` example. (#278)

For #277

I've been experimenting with `Ractor`s since upgrading to Ruby 3.0 for #275
but quickly ran into `Ractor::UnsafeError` when trying to call `Ox::sax_parse` in anything except the main `Ractor`.

Per Ruby's `Ractor` C extension documentation (link below):
"By default, all C extensions are recognized as `Ractor`-unsafe. If C extension becomes `Ractor`-safe,
the extension should call `rb_ext_ractor_safe(true)` at the `Init_` function and all defined method marked as `Ractor`-safe.
`Ractor`-unsafe C-methods only been called from main-ractor. If non-main ractor calls it, then `Ractor::UnsafeError` is raised."

I don't like to open seemingly-large feature requests like this without making some attempt at it myself first,
and luckily it seems like `Ox::sax_parse` Just Works™ since I marked it `Ractor`-safe, even with the `class_cache`.
Confirming this safety, making any remaining changes to `Ox::Sax`, and expanding this to the non-`Sax` parts of `Ox`
are all unfortunately out of my depth as a n00b C coder, so I would appreciate if you could take this over if it interests you.
I am happy with just `Sax` support since I have no current need for marshalling,
but I imagine other `Ox` users wouldn't be satisfied if stratified.

In this commit:
- Enable `rb_ext_ractor_safe` preprocessor macro via `have_func` in `extconf.rb`.
- Mark `Init_Ox` and `ox_sax_parse` as `Ractor` -safe.
- Add a new `Ractor`-based `Ox::Sax` example exercising both parallel and serial `Ox::Sax` handler `Ractor`s
  to parse data from `shared-mime-info` XML files many users likely already have on their systems.

Official `Ractor` info:

- "Ractor: a proposal for a new concurrent abstraction without thread-safety issues": https://bugs.ruby-lang.org/issues/17100 (ruby/ruby#3365)
- Ruby's official `Ractor` documentation: https://docs.ruby-lang.org/en/master/doc/ractor_md.html
- "A way to mark C extensions as thread-safe, Ractor-safe, or unsafe": https://bugs.ruby-lang.org/issues/17307 (ruby/ruby#3824)
- Ruby's C Extension `Ractor` documention covering `rb_ext_ractor_safe`: https://docs.ruby-lang.org/en/master/doc/extension_rdoc.html#label-Appendix+F.+Ractor+support
- A `Ractor` C Extension from the creator of `Ractor` that might serve as a useful example: https://github.com/ko1/ractor-tvar

Blogs:

- "Ractors: Multi-Core Parallel Processing Comes to Ruby 3": https://www.ruby3.dev/ruby-3-fundamentals/2021/01/27/ractors-multi-core-parallel-processing-in-ruby-3/
- "Ruby Ractor Experiments: Safe async communication" :https://ivoanjo.me/blog/2021/02/14/ractor-experiments-safe-async/
- "Playing with Ruby Ractors": https://billy-ruffian.co.uk/playing-with-ruby-ractors/
- "How Fast are Ractors?": https://www.fastruby.io/blog/ruby/performance/how-fast-are-ractors.html (https://github.com/noahgibbs/ractor_basic_benchmarks/tree/main/benchmarks)

Before this change:

```
[okeeblow@emi#CHECKING-YOU-OUT] time ./bin/checking-you-out ~/224031-dot-jpg
Received /home/okeeblow/.local/share/mime/packages/user-extension-rsrc.xml
/home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:375:in `sax_parse': ractor unsafe method called from not main ractor (Ractor::UnsafeError)
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:375:in `block in open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:331:in `open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival/mr_mime.rb:331:in `open'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:24:in `block (2 levels) in remember_me'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:19:in `loop'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:19:in `block in remember_me'
<internal:ractor>:583:in `send': The incoming-port is already closed (Ractor::ClosedError)
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:86:in `block in extended'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `each'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `each_with_object'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/ghost_revival.rb:63:in `extended'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/inner_spirit.rb:216:in `extend'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out/inner_spirit.rb:216:in `<top (required)>'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out.rb:10:in `require_relative'
        from /home/okeeblow/Works/DistorteD/CHECKING-YOU-OUT/lib/checking-you-out.rb:10:in `<top (required)>'
        from ./bin/checking-you-out:3:in `require_relative'
        from ./bin/checking-you-out:3:in `<main>'
./bin/checking-you-out ~/224031-dot-jpg  0.12s user 0.05s system 100% cpu 0.168 total
```

My new `Ox::Sax` Ractor example script's usage:

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb
Please provide the path to a `shared-mime-info` XML package and some media-type query arguments (e.g. 'image/jpeg')
```

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml
Please provide some media-type query arguments (e.g. 'image/jpeg')
```

Finding all-extant types:

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml image/jpeg font/ttf application/xhtml+xml image/x-pict
Parallel Ractors
["Worker 0 gave us JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]",
 "Worker 1 gave us TrueType 字型 (font/ttf) [.ttf]",
 "Worker 2 gave us XHTML 網頁 (application/xhtml+xml) [.xhtml,.xht]",
 "Worker 3 gave us Macintosh Quickdraw/PICT 繪圖 (image/x-pict) [.pct,.pict,.pict1,.pict2]"]

Serial Ractor
"ONLY ONE OX gave us [#<CYO JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]>, #<CYO TrueType 字型 (font/ttf) [.ttf]>, #<CYO XHTML 網頁 (application/xhtml+xml) [.xhtml,.xht]>, #<CYO Macintosh Quickdraw/PICT 繪圖 (image/x-pict) [.pct,.pict,.pict1,.pict2]>]"
```

…and not finding invalid ones:

```
[okeeblow@emi#ox] ./examples/sax_ractor.rb /usr/share/mime/packages/freedesktop.org.xml lol/rofl fart/butt image/jpeg
Parallel Ractors
["Worker 0 gave us nothing",
 "Worker 1 gave us nothing",
 "Worker 2 gave us JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]"]

Serial Ractor
"ONLY ONE OX gave us [nil, nil, #<CYO JPEG 影像 (image/jpeg) [.jpeg,.jpg,.jpe]>]"
```

Unit tests pass.