Remove non-interior mutability from non-generic types

[madsmtm]

Part of #563.

Remove non-interior mutability from the types NSString, NSAttributedString, NSData, and a few others. Remaining are only the generic types NSArray, NSSet, NSDictionary, NSEnumerator and so on, which will be handled by #587.

The move of NSString was facilitated by #628.

The changes to NSData might incur a performance cost if users decide to use the safe to_vec over the unsafe as_slice_unchecked, and the iteration methods are also less performant now.

Notes:

• Send+Sync was removed from all of the above types, as they're now interior mutable.
• The trait impls like std::io::Write that require &mut was changed to be implemented on &T instead of T. This could be further improved with different impls on Retained<T>.