Feature/red black trees

[remexre]

Changes

Adds sets, maps, and multimaps implemented directly in Silver.
This lets them share structure, which should reduce peak memory use and GC pressure.

This is on top of feature/docgen, so waiting on it to be merged first.

Documentation

• A comment on the implementation points to a better explanation than I can write of the deletion algorithm
• The basic RBT operations are as in Okasaki (I suppose I could put a page number etc there too)
• Doc comments are yet to be written, will do that before making this no-longer-a-draft
• I still need to implement join and split; I'll comment those once they're written

Testing

• Property tests would be worth doing?

Performance

• I'll start using these in Silver and do actual performance comparisons before this is merged.
• The case where I think this is an obvious win is synthesized attributes that are sets, which are currently (often) represented as linked lists.
• Eric and I briefly discussed having a more generic environment library; in principle, I'd assume that one could do one that saved a bunch of work with laziness.

TODOs

• join, split (for faster set operations)
• Write doc comments
• Use these in Silver
• Compare performance to treeset