py: Add MP_ROM_* macros and mp_rom_* types and use them.
diff --git a/py/runtime.h b/py/runtime.h
index 2282ea0..1449619 100644
--- a/py/runtime.h
+++ b/py/runtime.h
@@ -48,6 +48,7 @@
     bool u_bool;
     mp_int_t u_int;
     mp_obj_t u_obj;
+    mp_rom_obj_t u_rom_obj;
 } mp_arg_val_t;
 
 typedef struct _mp_arg_t {