Add notion of difference, useful for diffing queries

[Ericson2314]

Hopefully this eventually grows into supporting lattice (and related algebra) operations more fully.

[kmicklas]

This can already be built with mapMaybeV and a monoid operation, and I don't think it's any more efficient, so I'd prefer it as a separate function outside the View class.

[Ericson2314]

I'm not sure that's just as efficient. Beyond the intermediate datastructure, there's the fact that the function has to be run on every value in the new one. Difference only runs the function on elements in both; it can skip subtrees if it can tell they are not in the subtracted vessel. Not sure if the map data structures currently exploit this, but not describe ones (internal maps, etc) could probably do even better.