Ahaggar: add the TARGET_EXPR node walker

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: add the tree_walker variable to fix a compilation error

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: remove unused variables

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: add the ahg_ast_node_walker() helper

* This function will format the output and automatically call the node
  callback walker.

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