Rename "PhabricatorHash::digest()" to "weakDigest()"

Summary: Ref T12509. This encourages code to move away from HMAC+SHA1 by making the method name more obviously undesirable.

Test Plan: grep, browsed around.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12509

Differential Revision: https://secure.phabricator.com/D17632