py: Change type of .make_new and .call args: mp_uint_t becomes size_t.

This patch changes the type signature of .make_new and .call object method
slots to use size_t for n_args and n_kw (was mp_uint_t.  Makes code more
efficient when mp_uint_t is larger than a machine word.  Doesn't affect
ports when size_t and mp_uint_t have the same size.
diff --git a/py/objstr.h b/py/objstr.h
index d9cc85e..d9327b1 100644
--- a/py/objstr.h
+++ b/py/objstr.h
@@ -60,7 +60,7 @@
     else { str_len = ((mp_obj_str_t*)MP_OBJ_TO_PTR(str_obj_in))->len; str_data = ((mp_obj_str_t*)MP_OBJ_TO_PTR(str_obj_in))->data; }
 #endif
 
-mp_obj_t mp_obj_str_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args);
+mp_obj_t mp_obj_str_make_new(mp_obj_t type_in, size_t n_args, size_t n_kw, const mp_obj_t *args);
 void mp_str_print_json(const mp_print_t *print, const byte *str_data, size_t str_len);
 mp_obj_t mp_obj_str_format(mp_uint_t n_args, const mp_obj_t *args, mp_map_t *kwargs);
 mp_obj_t mp_obj_str_split(mp_uint_t n_args, const mp_obj_t *args);