Move PhutilProseDiff out of "libphutil/"
Summary: Ref T13414. Ref T13425. Ref T13395. Move this stuff to "phabricator/" since Arcanist is not likely to ever need it.
Test Plan: Grepped for "PhutilProseDiff".
Maniphest Tasks: T13425, T13414, T13395
Differential Revision: https://secure.phabricator.com/D20837