py: Add MP_ROM_* macros and mp_rom_* types and use them.
diff --git a/py/objstringio.c b/py/objstringio.c
index 1823ed3..d0c0a4f 100644
--- a/py/objstringio.c
+++ b/py/objstringio.c
@@ -142,15 +142,15 @@
     return o;
 }
 
-STATIC const mp_map_elem_t stringio_locals_dict_table[] = {
-    { MP_OBJ_NEW_QSTR(MP_QSTR_read), (mp_obj_t)&mp_stream_read_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_readall), (mp_obj_t)&mp_stream_readall_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_readline), (mp_obj_t)&mp_stream_unbuffered_readline_obj},
-    { MP_OBJ_NEW_QSTR(MP_QSTR_write), (mp_obj_t)&mp_stream_write_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_close), (mp_obj_t)&stringio_close_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR_getvalue), (mp_obj_t)&stringio_getvalue_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR___enter__), (mp_obj_t)&mp_identity_obj },
-    { MP_OBJ_NEW_QSTR(MP_QSTR___exit__), (mp_obj_t)&stringio___exit___obj },
+STATIC const mp_rom_map_elem_t stringio_locals_dict_table[] = {
+    { MP_ROM_QSTR(MP_QSTR_read), MP_ROM_PTR(&mp_stream_read_obj) },
+    { MP_ROM_QSTR(MP_QSTR_readall), MP_ROM_PTR(&mp_stream_readall_obj) },
+    { MP_ROM_QSTR(MP_QSTR_readline), MP_ROM_PTR(&mp_stream_unbuffered_readline_obj) },
+    { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
+    { MP_ROM_QSTR(MP_QSTR_close), MP_ROM_PTR(&stringio_close_obj) },
+    { MP_ROM_QSTR(MP_QSTR_getvalue), MP_ROM_PTR(&stringio_getvalue_obj) },
+    { MP_ROM_QSTR(MP_QSTR___enter__), MP_ROM_PTR(&mp_identity_obj) },
+    { MP_ROM_QSTR(MP_QSTR___exit__), MP_ROM_PTR(&stringio___exit___obj) },
 };
 
 STATIC MP_DEFINE_CONST_DICT(stringio_locals_dict, stringio_locals_dict_table);