WT node access, rank and select

[alexbowe]

Provides functions to access, rank, and select bits over a WT node (only certain kinds).

These are still only hand-tested, but at least they won't break any current functionality, and someone might find them useful, so I figured I should issue a pull request now.

(It's also my first non-trivial pull request, so I wanted to try it out).

[tb-robot]

Can one of the admins verify this patch?