Consider the `mp_obj_new_str_iterator` called once and `cur` is 0, maybe it's not necessary. But this is a copy of [mp_obj_new_tuple_iterator](https://github.com/micropython/micropython/blob/71c5181a8dfa69ba9f5ca322a3aba0660be2e166/py/objtuple.c#L181) and [mp_obj_new_list_iterator](https://github.com/micropython/micropython/blob/71c5181a8dfa69ba9f5ca322a3aba0660be2e166/py/objlist.c#L278), the `cur` looks more like a reserved argument.