Mostly align parser to PHP 5.5.8

Summary: Ref T4334. Adds support for: traits; yield; short array syntax; callable; finally.

Test Plan:
Unit tests. Attempted to use PHPAST web view to verify I didn't build the trees wrong.

{F103626}

Generally, these changes are difficult to verify by inspection, and errors only become clear once the tree is in use. So probably not everything is quite correct, but it should be relatively close.

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D8033