lang: add user args to ast iterator; combine pre/post-order traversal modes

the caller can now provide a pointer arg to ivy_ast_node_iterate, which will
be forwarded to the specified callback function each time it is called.

the iterator now behaves similarly to fts, in that it visits each node
in both pre-order and post-order, and indicates to the callback whether
the current iteration is pre- or post-order.

lang/ast/iterate.h

@@ -1,6 +1,9 @@
 #ifndef _AST_ITERATE_H_
 #define _AST_ITERATE_H_
 
+struct ivy_ast_node_iterator;
+struct ivy_ast_node;
+
 extern void ast_node_iterator_enqueue_node(
 	struct ivy_ast_node_iterator *it, struct ivy_ast_node *parent,
 	struct ivy_ast_node *node);