Add available! macro

This includes internal methods for looking up the OS version at runtime,
and for looking up the deployment target at compile time.

crates/objc2/CHANGELOG.md

@@ -24,6 +24,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * Allow using `Into` to convert to retained objects.
 * Make `Retained::into_super` an inherent method instead of an associated
   method. This means that you can now use it as `.into_super()`.
+* Added the `available!()` macro for determining whether code is running on
+  a given operating system.
 
 ### Changed
 * **BREAKING**: Changed how you specify a class to only be available on the

crates/objc2/src/__macro_helpers/mod.rs

@@ -1,6 +1,7 @@
 pub use core::borrow::Borrow;
 pub use core::cell::UnsafeCell;
 pub use core::convert::AsRef;
+pub use core::default::Default;
 pub use core::marker::{PhantomData, Sized};
 pub use core::mem::{size_of, ManuallyDrop, MaybeUninit};
 pub use core::ops::Deref;
@@ -21,6 +22,7 @@ mod method_family;
 mod module_info;
 mod msg_send;
 mod msg_send_retained;
+mod os_version;
 mod writeback;
 
 pub use self::cache::{CachedClass, CachedSel};
@@ -38,6 +40,7 @@ pub use self::method_family::{
 pub use self::module_info::ModuleInfo;
 pub use self::msg_send::MsgSend;
 pub use self::msg_send_retained::{MaybeUnwrap, MsgSendId, MsgSendSuperId};
+pub use self::os_version::{is_available, AvailableVersion, OSVersion};
 
 /// Disallow using this passed in value in const and statics for forwards
 /// compatibility (this function is not a `const` function).