Support "x bits in bytes" in phutil_units()

Summary: Ref T11140. I need to generate some encryption keys to do at-rest encryption, and the code can be a little more readable as 256 bits in bytes, etc.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11140

Differential Revision: https://secure.phabricator.com/D16123