Merge: nitpick, the nit lintian

Check Nit programs for issues and other stuff.

Currently, only a fast checker to use with the vim plugin.
50% faster than before.

~~~
$ time nitg nitg.nit --only-metamodel
user	0m1.752s
$ time nitpick nitg.nit
user	0m0.872s
~~~

Pull-Request: #724
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>