ahaggar-ast: improve the AST pretty printer

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
walk-nodes: remove the 'if' from the COND_EXPR walker

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
walk-nodes: add walk_cond_expr_then_node()

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
walk-nodes: add walk_cond_expr_else_node()

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