Ahaggar: add helper to print VAR_DECL and PARM_DECL

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: add more node walkers

* Array domain walker
* cond expr walker
* binary arith walker

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: add the walk-nodes.c file

This file contains the definition of all the walkers used by Ahaggar
to traverse trees.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>