Worked on Object Type while documenting it on the wiki.
- Renamed $object_type->object_type property to $class.
- Removed has_subtype element from  array of WP_Object_Type::$_core_object_types.
- Renamed WP_Object_Type::$_core_object_types to $_object_type_classes
- Renamed $object_type->register_object_type() to register_class()
- Added register_object_type_class() function.
- Renamed $object_type->assign_type() to assign()
- Removed option to pass a non-qualified object type class to $object->assign_type() to avoid edge case ambiguity since custom classes can be registered.
- Changed all references of 'all' subtype to be 'any' and initialized empty subtypes to have a value of `any'.
- Fixed code for $object_type->is_equivalent()