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

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: FlatStrings built from NativeString now have free to_cstring.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: Fix the case of null items in FlatBuffer when calling to_s

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
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>