stdlib : Cleaned the code for the Strings, renamed attributes to be compliant with Nit's naming policy.
Also added more comments for a better lisibility.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
stdlib: Strings, added function to avoid passing refs with a NativeString (can cause bugs if auto/manually deallocated with FFI).

Signed-off-by: Lucas Bajolet <lucas.bajolet@hotmail.com>
stdlib : String, added test for copy constructor

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