Merge: Nit Actor Model, with some examples

A better version than the previous PR !

The model injection and support module generation are now two phases in the frontend.

Needs #2357 to be able to compile with `nitc`, for now you have to use it as `nitc module.nit -m actors_module.nit` for compiling.

Pull-Request: #2361
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>