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/app_state.rs

@@ -16,12 +16,12 @@ use core_foundation::runloop::{
     kCFRunLoopCommonModes, CFRunLoopAddTimer, CFRunLoopGetMain, CFRunLoopRef, CFRunLoopTimerCreate,
     CFRunLoopTimerInvalidate, CFRunLoopTimerRef, CFRunLoopTimerSetNextFireDate,
 };
-use icrate::Foundation::{
-    CGRect, CGSize, MainThreadMarker, NSInteger, NSOperatingSystemVersion, NSProcessInfo,
-};
 use objc2::rc::Id;
 use objc2::runtime::AnyObject;
 use objc2::{msg_send, sel};
+use objc2_foundation::{
+    CGRect, CGSize, MainThreadMarker, NSInteger, NSOperatingSystemVersion, NSProcessInfo,
+};
 
 use super::uikit::UIView;
 use super::window::WinitUIWindow;