Evaluate joblib as a replacement for caching

[tyarkoni]

Back when caching was implemented, joblib had some pickling issues that led to us creating our own in-memory memoization. Apparently joblib now works okay with dill and should have less trouble pickling arbitrary objects (though this needs some verification), so we should try to replace our current caching approach with a joblib-based solution.

[tyarkoni]

See #95