RCR 306: include rbtree in the stdlib

A good red/black tree implementation in the standard library would enable several algorithms to be implemented more efficiently.

There is, as yet, no efficient implementation of data structures such as a priority queue and a sorted collection in ruby's standard library. This often leads to algorithmically suboptimal but implementationally convenient reinventions of the wheel.

Add a red/black tree implemetation to the standard library, and classes like a Priority Queue and Sorted List as interfaces to it.

While there's a red/black tree available in the RAA, a balanced tree is a fundamental enough data structure that one should be available by default.

There's already one at