Using obj_id in classext_cow_classes is slow and also is a problem when freeing classes because the lookup requires the EC, which is not available if it is running on a GC thread. This will cause it to crash on MMTk. We don't need to use the obj_id as the key in the classext_cow_classes st_table because the class itself is pinned and won't move so we can use it as the key instead.