Update objc2 crates (#3634)

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.

src/platform_impl/ios/view_controller.rs

@@ -1,8 +1,8 @@
 use std::cell::Cell;
 
-use icrate::Foundation::{MainThreadMarker, NSObject};
 use objc2::rc::Id;
 use objc2::{declare_class, msg_send_id, mutability, ClassType, DeclaredClass};
+use objc2_foundation::{MainThreadMarker, NSObject};
 
 use super::app_state::{self};
 use super::uikit::{