[pfff] support for trait constraints

Test Plan:
unit test included

also $ ~/pfff/pfff -dump_php traits_constraint.php
=> https://phabricator.fb.com/P4904226

Reviewers: eletuchy, glevi, jwatzman, julienv, jcreed

Reviewed By: eletuchy

CC: bryano, amenghra

Differential Revision: https://phabricator.fb.com/D1146956