Merge: lib/standard: reimplement `files` with light FFI

So I can use it during the bootstrap (thanks #1323)

Pull-Request: #1363
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>