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.

It'd also be nice to have similar conversions from NSArray<NSView> to NSArray<NSResponder>.

Note that this is only valid for __covariant generic types, e.g. NSArray, but not NSMutableArray (see also this video from WWDC2015, timestamp around 23:40).

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.