Make Id a "fundamental" type (like Box)

This changes the coherence rules to give Id less flexibility in terms of forwards compatibility, but in return allowing downstream crates (like objc2_foundation) to implement From for this type.

objc2/src/lib.rs

@@ -65,6 +65,7 @@ The bindings can be used on Linux or *BSD utilizing the
 )]
 #![warn(missing_docs)]
 #![allow(clippy::missing_safety_doc)]
+#![feature(fundamental)]
 // Update in Cargo.toml as well.
 #![doc(html_root_url = "https://docs.rs/objc2/0.2.7")]
 

objc2/src/rc/id.rs

@@ -92,6 +92,7 @@ use crate::Message;
 /// let cloned: Id<T, Shared> = shared.clone();
 /// // Do something with `&T` here
 /// ```
+#[fundamental]
 #[repr(transparent)]
 // TODO: Figure out if `Message` bound on `T` would be better here?
 // TODO: Add `?Sized + ptr::Thin` bound on `T` to allow for extern types