Minor tidying of XHPAST code

Summary: Fix a few linter violations in XHPAST source code and modify a bunch of code for consistency.

Test Plan: Ran arc unit --everything in rARC, rPHU and rP.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13987