Soundness fixes

[madsmtm]

• Properly mark objects as mutable behind shared references.
• Fix auto trait impls, see Tracking Issue for sound builtin auto trait impls rust-lang/rust#93367.

[madsmtm]

Didn't really realize this before, but making type OpaqueData = UnsafeCell<...> actually means that Id<T, Shared> is much safer!

Having aliasing Id<T, Owned> and Id<T, Shared> is ofc. still UB, but if you only use Id<T, Shared> and don't implement Send and/or Sync on your type then Objective-C methods are free to mutate the object as much as they want!

(Concretely, this removes a few noalias and readonly attributes from the LLVM IR of e.g. &Object and &Class, just like using an extern type for those would)

[madsmtm]

To consider: Does it make sense for Id to store an UnsafeCell? How about variance?

EDIT: Nope, it's correct as-is.

[madsmtm]

Also, the pointer in Id<T, Shared> may actually have been mutated by Id::clone, so making Object = UnsafeCell is definitely better!