Pfff parser fix: collection literals now with less fail

Summary:
Moved stuff into the parser; reuse the array stuff
and convert the AST nodes inline.

Test Plan:
* output of `./pfff -dump_php test.php` is unchanged from D700450
* `make test`  still shows an objc failure; no longer shows php failure
* manually verified `parser_php.output`; no new conflicts

Reviewers: pad, andrewparoski

Reviewed By: pad

CC: ps

Differential Revision: https://phabricator.fb.com/D701245