pfff: allow 'function f1( (function(string):int) $callback)' syntax

Test Plan:
unit test included
make test

Reviewers: julienv, amenghra, pieter

Reviewed By: pieter

CC: pieter, zoel, lisp, michalburger1

Differential Revision: https://phabricator.fb.com/D723875
* tests/php/parsing/type_hints_higher_order.php: use hh, otherwise xhpast choke