Provide dispose() method for XHPASTTree.

Summary:
PHP's "GC" doesn't do cycle detection so you can't get currently rid of these.
This creates a lot of memory pressure in Diviner (and maybe lint).

Test Plan:
Updated Diviner to use this method and have a reasonable peak memory use.

Reviewed By: jungejason
Reviewers: aran, jungejason, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 205