Redo how ownership works

[madsmtm]

Add associated type ClassType::Mutability, which describes the mutability of an object. This replaces the Ownership parameter in Id<T, O> (so we're moving the mutability/ownership specification from use site to declaration site, which is a bit more restrictive, but much more succinct, as well as being easier to understand and harder to make mistakes).

Along with this, we can also finally fix NSCopying and NSMutableCopying in a way that ProtocolObject<dyn NSCopying> is allowed.

The rough idea was initially described in #399 (comment), see the code and changelog for details on how the exact design turned out.

• Fixes Making Id::retain safer #399 by creating ClassType::retain, which is safe for certain classes.
• Fixes Improving mapping of generic types #316 by removing the Ownership type on generics.
• Fixes Fix NSCopying and NSMutableCopying #401 by removing the associated types from NSCopying and NSMutableCopying, and instead having that data encoded in the associated type ClassType::Mutability.
• Fixes The deal with mutability #265 by disallowing Id<T> -> &mut T when T is not mutable. The remaining part in that (proper main thread safety) is tracked in Sendability attributes #359.

[madsmtm]

Which kind of subclassing should be allowed (A -> B means kind A may have superclasses with kind B):

Unknown -> *

Immutable -> Immutable

ImmutableWithMutableSubclass -> ImmutableWithMutableSubclass + Mutable
Mutable -> Mutable

InteriorMutable -> InteriorMutable + MainThreadOnly
MainThreadOnly -> MainThreadOnly

[madsmtm]

I'm wondering how to assign things in icrate, and I actually think we don't really need Unknown, rather it should be called Root, and instead we should just default everything to InteriorMutable (except of course for the things that implement NSMutableCopying)?

[madsmtm]

Also, InteriorMutable and Immutable are currently equivalent? (apart from Send + Sync, but those should be implemented separately)

Though perhaps we can still use it in e.g. NSDictionary to only allow Immutable keys, thereby fixing #337?

[madsmtm]

I considered that it may be favourable to allow changing the class kind from the default to Mutable later on, and would like to make that a non-breaking change, but since we'd still need to change the methods from &self to &mut self, there is not really much value in that (e.g. might be relevant for NSFormatter?)

Changing from the default InteriorMutable to Immutable is safe though!

[madsmtm]

Tagging @simlay, @silvanshade and @mwcampbell as you might want to be notified of this change: Id no longer has an ownership parameter O, instead the information about ownership/mutability is moved to an associated constant ClassType::Mutability, which should feel significantly simpler to use.

The changelog should contain migration instructions, but please let me know if you run into any issues!

The change is primarily done because people (including myself) keep wanting to use &mut self in declare_class!, which is almost always unsound when overriding AppKit classes or implementing delegates - instead one should use &self + interior mutability (instance variables wrapped in Cell).