We have to support self.used. This part of code was disabled
because Pyston didn't support co_names. Now enable it. But Pyston's
bytecode is different than CPython bytecode. So we can't use the code
which rely on co_code part.

The last variable in the return list is the code.names field.
After some experiments, seems return an empty value has no side
effects, at least for now. Let's see....