Use objc2-metal

[andyleiserson]

Description taken from #5641, which also has a bunch more background and context I'm not duplicating here. I'm creating an issue for tracking in our project board.

Use the objc2-metal crate instead of the metal crate.

This will:

• Improve memory management and soundness
• Make it easier to quickly support new Metal APIs (they're either already generated for you, or is basically just an update of Xcode away).
• Possibly allow reducing the usage of Arc, as Metal objects are already reference-counted.
• Likely improve performance, we use objc_retainAutoreleasedReturnValue underneath the hood to avoid putting objects into the autorelease pool when possible, reducing memory pressure.
• Make it possible to properly support Add tvOS support #5578 and visionOS.

[jimblandy]

Yes, let's re-triage #5641 when we get to this.