Add an "OpaqueEnvelope" for passing passwords around

Summary: See T1526. To prevent passwords from appearing in logs and traces, provide an object which can wrap them up.

Test Plan: Ran unit tests.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1526

Differential Revision: https://secure.phabricator.com/D2991