Reverted Identifier trait.
Added Ownership types.
Added stricter type bounds on Id.
Made IdVector traits generic over ownership.
Added Ownership type param to INSArray.
Added Ownership type param to INSDictionary.
Added Owned and Shared array traits.
Split out an object_impl macro.
Added an Ownership type param to NSArray.
Added NSMutableArray trait.

Can't test it yet because I can't implement DerefMut for Id without
breaking everything...