A low-overhead Zig interface for Metal.

• Following metal-cpp extensions for API coverage
• Example apps based on https://developer.apple.com/metal/LearnMetalCPP.zip
• Translating objective-c with minimal Zig changes (maybe auto-generated later?)
• Opaque types as strongly-typed wrappers for classes
• Use generics with mixins to implement method inheritance
• Enumerations as integer values
• Selector name as function name with : -> _ and trailing underscore removed
• Namspace prefix removed (MTLDevice -> mtl.Device)
• Adds a cast method to ns.Object that provides some static safety guarantees

• Optional vs errors for null values
• Const doesn't seem to matter for opaque types?
• Zig slices or pointer/count (e.g. array initialization from objects)
• String constants (e.g NSErrorDomain)
• Delegate lifetimes
• Why do function parameters get warning about shadowing struct fields