Use objc_id to handle managing Objective-C objects

[gfx-rs]

Use objc_id to handle managing Objective-C objects

[JohnColanduoni]

Metal returns a lot of unretained pointers in hot paths (via ObjC's "get" rule) to avoid unnecessary retain/release cycles and using objc-id for all the types would make this difficult. For example, ToOwned could not be implemented for a Metal type T since objc-id does not implement Borrow<T>.

The foreign_obj_type! is just a convenience wrapper over foreign_type! from the foreign_types crate. It is used in most of the Core* bindings from the Servo team, which was the impetus for using it here.

[mjadczak]

Seems this has been answered, @jrmuizel can you close it if that makes sense to you?

[jrmuizel]

Yeah, this makes sense.