Implementation of PhutilLexer for Python

Summary: This appears to be ~1.5-2.0x faster than pygments on my machine, which isn't as good as I was hoping for but will help. (This is ~330 ms for a 4400-line file I chose at random.)

Test Plan: Loaded a Python paste and saw highlights that approximately (or maybe exactly) match the pygments version.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9370