Add previous and next sibling pointers to AASTNode

Summary: Adds AASTNode::$previousSibling and AASTNode::$nextSibling properties. The primary use case is for writing a custom linter rule.

Test Plan: Tested in my custom linter rule.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D14496