WIP for struct/enum
See also: https://github.com/kmcallister/servo/commit/c20b50bbbbcdc8ce3551adbc1e039a727cf89995

Todo:
 - Make this work for function return values (allow if fn is unsafe)
 - Make this work for function args
 - Improve lint messages
 - Have this check stuff like Option<JS<T>> too. Either by iterating through the tree of type params (expensive), blacklisting Option<T>/RefCell/etc, or using check_ty (might not work)