Merge: Faster buffers

Make flatbuffers a little faster on substrings

for nitc/nitc/nitc
before: 0m7.168s
after: 0m7.068s (-1.4%)

Pull-Request: #1189
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc lib/standard/stream.nit
Simple merge

diff --cc lib/standard/string.nit
Simple merge