Expose xhpast via Conduit
Summary:
Create phpast.{version,getast} methods for calling xhpast
with --version and with source code as input, respectively.
Test Plan:
Run arc call-conduit a bunch of times; delete xhpast; run
it a bunch more times.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T1534
Differential Revision: https://secure.phabricator.com/D3289