Header only

See merge request !22

New way of (de)serialization of objects into TNL::File using operator<< and operator>> (starting with string)

This is nicer and safer (thanks to exceptions) than save/load functions.
Moreover, it breaks the last cyclic dependency between TNL classes.

Return values are nicer than modifiable parameters. This also avoids the
circular dependency between TNL::String and List.

This is much simpler than using the C functions. As a bonus, I/O errors
throw exceptions.