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)
En fait, c’est exactement ce que fait [OpenJDK](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/StringBuilder.java#StringBuilder.%3Cinit%3E%28%29).