Add Metal support

[jgilchrist]

Used Blightwidow's attempt as a starting point, but have:

• Ported to objc2 and replaced the ad-hoc bindings to MPS with objc2-metal-performance-shaders
• Refactored significantly to try and move as much metal-specific logic into metal.rs or Dialect among other cleanups and simplifications

I've left the commit history untouched (since the objc2 port which I posted about in the linked issue) to give an idea of the changes, but will squash if things look ok.

There's a much larger potential refactoring (splitting all the Metal stuff into bullet-metal and the common code into bullet-compiler as @jw1912 said in #500) which I haven't tackled, but let me know if you feel like this is a pre-requisite for merging.

Fixes #500

[jw1912]

Looks good! I have some nitpicks - mostly about imports

[jgilchrist]

Do you want me to try adding a CI job for this? I don't think the macOS runners have Metal, but we still might be able to run check

[jw1912]

Do you want me to try adding a CI job for this? I don't think the macOS runners have Metal, but we still might be able to run check

Maybe in a separate PR?

[jw1912]

If you fix that clippy error by using i instead of buf_index, the only thing left would be to add some basic directions to https://github.com/jw1912/bullet/blob/main/docs/2-getting-started.md

[jgilchrist]

If you fix that clippy error by using i instead of buf_index

Fixed that and the 'too many args' one, those functions were still there because of the original change having those ad-hoc bindings and could be inlined.

the only thing left would be to add some basic directions to https://github.com/jw1912/bullet/blob/main/docs/2-getting-started.md

There's not a lot to do as everything should 'just work' after enabling the feature. But I've added that step anyway (plus it's the only place the available backends are listed I believe.

[jw1912]

Nice! And just to confirm if you run cargo test --workspace --features metal all the tests pass?

[jgilchrist]

@jw1912 Yep, all green!