Before this commit, classes and modules would be registered with the VM's `defined_module_hash`. The key was the ID of the class, but that meant that it was possible for hash collisions to occur. The compactor doesn't allow classes in the `defined_module_hash` to move, but if there is a conflict, then it's possible a class would be removed from the hash and not get pined. This commit changes the key / value of the hash just to be the class itself, thus preventing movement.