Merge: introduce Nith, the ligHt Nit compiler

It is just nitg with less features, thus should be faster to compile and should compile faster.
The result of the compilation should be the same however.

Independently, the series implements phases for the various nitg engines

Pull-Request: #385
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/abstract_compiler.nit
Simple merge

diff --cc src/separate_compiler.nit
Simple merge