Add dummy bytes() constructor.

Currently, MicroPython strings are mix between CPython byte and unicode
strings. So, conversion is null so far. This dummy implementation is
intended for compatibility with CPython (so, same code can run on both).
diff --git a/py/builtin.h b/py/builtin.h
index 050a216..4257de5 100644
--- a/py/builtin.h
+++ b/py/builtin.h
@@ -5,6 +5,7 @@
 MP_DECLARE_CONST_FUN_OBJ(mp_builtin_abs_obj);
 MP_DECLARE_CONST_FUN_OBJ(mp_builtin_all_obj);
 MP_DECLARE_CONST_FUN_OBJ(mp_builtin_any_obj);
+MP_DECLARE_CONST_FUN_OBJ(mp_builtin_bytes_obj); // Temporary hack
 MP_DECLARE_CONST_FUN_OBJ(mp_builtin_callable_obj);
 MP_DECLARE_CONST_FUN_OBJ(mp_builtin_chr_obj);
 MP_DECLARE_CONST_FUN_OBJ(mp_builtin_divmod_obj);