lang: ast: implement ast iteration

iteration is implementing without recursion, instead using type-specific callbacks to construct a queue of nodes to iterate through. ast priting is implemented using this functionality.

lang/ast/iterate.h

@@ -0,0 +1,9 @@
+#ifndef _AST_ITERATE_H_
+#define _AST_ITERATE_H_
+
+extern void ast_node_iterator_enqueue_node(
+	struct ivy_ast_node_iterator *it,
+	struct ivy_ast_node *parent,
+	struct ivy_ast_node *node);
+
+#endif