py/persistentcode: Change "len" type to size_t for mp_obj_str_get_data.

py/persistentcode.c

@@ -560,7 +560,7 @@ STATIC void save_obj(mp_print_t *print, mp_obj_t o) {
         } else {
             obj_type = 'b';
         }
-        mp_uint_t len;
+        size_t len;
         const char *str = mp_obj_str_get_data(o, &len);
         mp_print_bytes(print, &obj_type, 1);
         mp_print_uint(print, len);