nitg: add support for unary - as an extern method

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: nitg: add support for unary - as an extern method

Fixes #483.

TODO: Since "unary -" is an overload of "-", it cannot be declared as a callback from C. We could either extend the importation syntax or import both "-" and "unary -" on import of "-".

Pull-Request: #486
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>