WIP: Persistent Red Black Tree
Improve tests: assert Red Black properties and immutability always
WIP (broken): delete and immutability
Functional, but very slow Persistent Red Black Tree.

Pretty direct rendition of CLR's RBTree, with a Translator enum that
reverses left and right for symmetric cases and mutable wrapper nodes
providing parent and nil-sentinel for actual nodes.
Merge remote-tracking branch 'origin/master' into persistent-rb-tree
WIP on red black tree and generalizing internals
Clojure-like algorithm for Red Black Tree insertion. WIP on delete.

Supports bulk inserts in similar manner as PersistentMap.
WIP on delete
Assoc and dissoc for Persistent Red Black Tree. WIP.

TODO: iteration, min, max, sorted set, improve architecture.