Newstreams

Small refactoring of Streams/Files:
- Added a simple error management system
- Removed FDStreams, reified under FStream

Next PR to come: Proposition for renaming of the classes of the stream hierarchy (as requested in #466)
> If buffer is a null pointer, the function automatically allocates a buffer (using size as a hint on the size to use).
> — http://www.cplusplus.com/reference/cstdio/setvbuf/

So, in addition to @privat comment, I would add that `bf` is not really necessary here.