Merge: Parallelization phase improvement

Adds the support of methods with return value

If you use `is threaded` on a method with a return, it will instead return you a custom instance of `Thread` class on which you can use the method `join` to retrieve the return of the annotated method. `join` waits for the method to end before returning.If you want to make sure that the method has done his work before trying to retrieve it's return value, you can check it with the boolean `is_done`

Pull-Request: #1278
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc src/frontend/parallelization_phase.nit
Simple merge