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 add some optimizations on nitg-s and nitg-e engines

Some numbers with nitg and nith as a bench

~~~
$ nitg nitg.nit
$ time nitg nitg.nit
user	0m12.424s

$ nitg nith.nit
$ time nith nitg.nit
user	0m10.472s

$ nitg --erasure --inline-coloring-numbers nitg.nit
$ time nitg nitg.nit
user	0m11.672s

$ nitg --erasure --inline-coloring-numbers nith.nit
$ time nith nitg.nit
user	0m10.296s

$ nitg --erasure --rta --semi-global nitg.nit
$ time nitg nitg.nit
user	0m10.004s

$ nitg --erasure --rta --semi-global nith.nit
$ time nith nitg.nit
user	0m8.528s
~~~