Add PhabricatorHash::digestForIndex()

Summary: Does this seem reasonable? It's a bit more compact than digest() (6 bits / byte instead of 4 bits / byte) and 72 bits of entropy @ 12 bytes instead of 128 bits of entropy @ 32 bytes. I feel like it's important to preserve the printability, though, and this seemed like a fairly good balance of concerns.

Test Plan: unit tests

Reviewers: vrana

Reviewed By: vrana

CC: aran, yemao932

Differential Revision: https://secure.phabricator.com/D4253