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.rs

@@ -1,12 +1,12 @@
 #![allow(clippy::unnecessary_cast)]
 use std::cell::RefCell;
 
-use icrate::Foundation::{CGFloat, CGRect, MainThreadMarker, NSObject, NSSet};
 use objc2::rc::Id;
 use objc2::runtime::AnyClass;
 use objc2::{
     declare_class, extern_methods, msg_send, msg_send_id, mutability, sel, ClassType, DeclaredClass,
 };
+use objc2_foundation::{CGFloat, CGRect, MainThreadMarker, NSObject, NSSet};
 
 use super::app_state::{self, EventWrapper};
 use super::uikit::{