Add unpack support to xhpast

Test Plan:

cd support/xhpast
make cleanall
make
cd ../../
scripts/build_xhpast.php
arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15833