Changes relevant to Winit:
- `icrate` has been deprecated in favour of separate crates per
framework, in our case `objc2-foundation` and `objc2-app-kit` (and in
the future `objc2-ui-kit` on iOS).
- Moved `MainThreadMarker::run_on_main` to free-standing function
`run_on_main`.
- Changed how features work, this should result in less code that we
need to compile.
- Enums are now real structs instead of type-aliases and free constants.
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