Merge: lib/standard/string: Introducting Copy-on-Write FlatBuffers

As requested, implemented a Copy-on-Write mechanism on FlatBuffer, small performance improvement on the to_s operation.

Before :
![cct](https://cloud.githubusercontent.com/assets/1444825/4273670/fef3f1fc-3cec-11e4-9f81-8afaa6e4c92a.png)

After :
![concat](https://cloud.githubusercontent.com/assets/1444825/4273675/028c5bf6-3ced-11e4-93a9-ef20e94ba55f.png)

Pull-Request: #749
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>

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

diff --cc lib/string_experimentations/utf8.nit
Simple merge