Nith the new highly-efficient Nit compiler

Great things first:

before:

~~~
$ time ./nitg nitg.nit
real	0m11.181s
user	0m11.052s
sys	0m0.388s
~~~

after:

~~~
$ time ./nith nith.nit
real        0m4.000s
user        0m3.816s
sys 0m0.288s
~~~

see commits for details