Add safe upcasting

[madsmtm]

We already have the ability to convert from NSView to its superclasses NSResponder using Deref, ClassType::as_super or Id::into_super. In general, we should maybe consider an UpCast trait of some sort, to be able to do this fully generically? Even better if it could integrate with a From implementation for Id.

See also the inverse of this in #474.

[madsmtm]

The trait wasm_bindgen::JsCast shows some of the functionality that people seem to expect from this.

[madsmtm]

Also valuable from wasm_bindgen: They don't implement Borrow, BorrowMut nor AsMut, I've asked about the former here.

[madsmtm]

Tried implementing From for Id, but we can't do that generically as it conflicts with the reflexive From<T> for T; so we'll probably need FromId and IntoId traits :/ (see #48 previously).

Perhaps an inherent method on Id to access such traits could still make sense? Do we want to change Id::into_super to that? Maybe call it Id::upcast, and deprecate Id::into_super?

[madsmtm]

Some sort of UpCast trait (or maybe SubclassOf) could also be used as a bound when implementing external categories. It would allow us to emit something like:

extern_category!(
    unsafe trait MyCategory { ... }
    
    unsafe impl<T: SubclassOf<MyType>> MyCategory for T {}
);

Which would make it such that all subclasses of a given type MyType would now implement MyCategory.

[madsmtm]

Would also like to figure out how Id's and class type's impls of AsRef/Borrow (and mutable variants) should be. See also https://internals.rust-lang.org/t/semantics-of-asref/17016 for discussion around these traits.

[madsmtm]

The impl<T: ..., U> From<Retained<U>> for Retained<T> is, as outlined in #518 (comment), not possible. But we can do impl<T, U: AsRef<T>> for From<&U> for Retained<T>, so that's what I've done in 84152d8. Combined with f51c2e3, this should make .into() "just work", albeit with the tiny performance cost of possibly an extra retain/release. In cases where the user doesn't want a retained reference back, .as_ref() is the way to go.

I believe this is nicer than a generic UpCast trait and a .into_bikeshed(), since it means there's fewer traits and methods for users to learn about for a case that will be very common.

.into_super() is also possible now since it's an inherent method, see b94961a, so that should make it easier for people that don't want the perf cost / want to be explicit about the inheritance chain.