Merge: Faster write

Allow FlatString to be directly written to files without useless copy (`to_cstring`) or useless iterator (`substrings`)

Numbers, for `nitc nitc.nit`:

* before 5.796
* after 5.440 (-6%)

Close one check-box of #1298

Pull-Request: #1572
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>

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

diff --cc src/interpreter/primitive_types.nit
Simple merge