Merge: lib/std/file: do not flush on each write

Flushing for each small element of an output made things too slow.

real (not user) time for nitc/nitc/nitc:
before: 0m7.373s
after: 0m5.901s (-20%)

Pull-Request: #1216
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>