Upcasting generic collections

[madsmtm]

It'd be nice to have conversions to superclasses in collection types that use lightweight generics, i.e. some way to turn NSArray<NSView> into NSArray<NSResponder>, or even NSArray<NSObject>.

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

Related to #518 and might require a trait like in #271.

Also desirable: Some unsafe way to cast only the generics, e.g. NSArray<T>::cast_objects_unchecked<U>(self) -> NSArray<U>.

[madsmtm]

I added a fn as_opaque(&self) -> &CFArray<c_void> helper to CoreFoundation in 8a2f392, but that is of course still limited to only the most common case (i.e. it doesn't work generically).

[madsmtm]

In CoreFoundation's case, it would also be very useful to have some way to unsafely downcast the generics, e.g. from CFDictionary to CFDictionary<CFString, CFType>.

An alternative there would be to change the default generics to CFType, though that is potentially dangerous, since CF collections are not limited to only containing CoreFoundation types. Another alternative could be to change auto-generated function args/return types to CFDictionary<CFType, CFType>. But it's likely I'll have to audit that stuff manually.

[madsmtm]

Would also be nice to have this kind of casting work with toll-free bridging, i.e. allowing the user to go from NSArray<NSString> -> NSArray<CFString>.