Revert using __slots__ for Mapping subclasses in xray.utils

This recently added some complexity for a very nominal speed benefit. And
it appears that it breaks joblib serialization, somehow (even though
pickle works). So for now, revert it -- and consider filing a joblib bug
if we can narrow it down.
Merge pull request #166 from shoyer/dont-use-__slots__

Revert using __slots__ for Mapping subclasses in xray.utils