Add "PhutilProseDiff" classes to "phabricator/"

Summary: Depends on D20836. Ref T13414. Ref T13425. Ref T13395. Move these to "phabricator/" before trying to improve the high-level diff engine in prose diffs.

Test Plan: Ran "arc liberate", looked at a prose diff (no behavioral change).

Maniphest Tasks: T13425, T13414, T13395

Differential Revision: https://secure.phabricator.com/D20838