py: Declare help, input, open builtins in core.
These are *defined* per-port, but why redeclare them again and again.
diff --git a/py/builtin.h b/py/builtin.h
index 35d8910..f63eac7 100644
--- a/py/builtin.h
+++ b/py/builtin.h
@@ -71,6 +71,10 @@
MP_DECLARE_CONST_FUN_OBJ(mp_builtin_round_obj);
MP_DECLARE_CONST_FUN_OBJ(mp_builtin_sorted_obj);
MP_DECLARE_CONST_FUN_OBJ(mp_builtin_sum_obj);
+// Defined by a port, but declared here for simplicity
+MP_DECLARE_CONST_FUN_OBJ(mp_builtin_help_obj);
+MP_DECLARE_CONST_FUN_OBJ(mp_builtin_input_obj);
+MP_DECLARE_CONST_FUN_OBJ(mp_builtin_open_obj);
MP_DECLARE_CONST_FUN_OBJ(mp_namedtuple_obj);