nitg: Overridden gperf since it was slow to generate.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
nitg: Removed references to gperf in documentation and code

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge: Gperf fix

Since gperf was slow, better replace it with a slow to call solution at
runtime but fast to generate, it's not like we'll be generating
stacktraces often in the life cycle of a program.

Surprisingly enough, even on nitg, the stacktrace does not take long
to generate, which is a good thing.

For comparison :

~~~
$ time nitg nitg.nit --stacktrace libunwind
user	0m19.720s

$ time nitg nitg.nit --stacktrace nitstack
user	0m19.820s
~~~

Pull-Request: #387
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-g: use nitstack by default

Signed-off-by: Jean Privat <jean@pryen.org>