Disallow mutable generics in hashing collections

[madsmtm]

Continuation of #337, in #505 we restricted the key types to HasStableHash types, but types like NSArray<NSView> are still incorrectly considered stably hashable.

[madsmtm]

Maybe we should just make HasStableHash a manual trait, instead of having it be implemented automatically based on ClassType::Mutability?

[madsmtm]

Also somewhat related to #474, where we also have to have special behaviour for generic types.

[madsmtm]

I looked at the classes in Apple's frameworks that is used as the T inside NSSet<T> or the K inside NSDictionary<K, V>; it's a non-trivial amount of things (even if we discount type-aliases to NSString), so I think we have to do this automatically.

Maybe an approach could be to generate implementations based on whether the type is used anywhere as such? Since that's very strong evidence that the type is valid to use in a hashing collection.