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