AST printing callbacks: pass isl_ast_print_options

AST printing callbacks: pass isl_ast_print_options

The callback may want to pass control back to isl_ast_node_*_print,
for which it needs an isl_ast_print_options object.  Since the
caller of the callback already has a reference to such an object,
we might as well pass it along to the callback.

Since the callback may want to modifiy the isl_ast_print_options
object, we turn it into a properly reference counted isl object.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>