Merge: Better nitcc

Some improvements and bugfixes for nitcc.

* Improve code size in C and in Nit
* Produce valid Nit code even in case of R/R, S/R or S/R/R conflicts. Close #1693

Before: compiling objc_test_parser on an old laptop
* 880.58 usertime
* 3GB memory
* produced executable: 50MB (31MB stripped)

after: same grammar, better nitcc
* 337.80usertime
* 788MB memory
* produced executable: 28MB (8,8MB stripped)

Pull-Request: #1694
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>