Merge: Robust IO

Some cleanup on lib/standard/file.nit
Especially close #755

There is still some work to do and question to answer from an API point of view.

Pull-Request: #1246
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>