Red-black tree

This is a basic implementation of a red-black tree in F#, which is pretty close to being correct by construction.

Things to fix

• Can we prove that the failwith cases indeed will never be hit? (Is it even true?)
• Can we do better than the awful recursion to elevateBlack and elevateRed? That is, can we make the type parameter truly phantom, erased at runtime?
• Can we get some constraints at the type level to help us get the values assigned to the various nodes in the right order?