objstr: Add TODO-optimize for using .join with arbitrary iterable.

py/objstr.c

@@ -330,6 +330,7 @@ STATIC mp_obj_t str_join(mp_obj_t self_in, mp_obj_t arg) {
     } else {
         if (!MP_OBJ_IS_TYPE(arg, &mp_type_list)) {
             // arg is not a list, try to convert it to one
+            // TODO: Try to optimize?
             arg = mp_type_list.make_new((mp_obj_t)&mp_type_list, 1, 0, &arg);
         }
         mp_obj_list_get(arg, &seq_len, &seq_items);