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/node.h

@@ -17,6 +17,8 @@ struct ast_node_type {
 		struct ivy_ast_node *, struct ivy_ast_node *);
 	void (*n_print)(struct ivy_ast_node *);
 	void (*n_init_state)(struct parser_state *);
+	void (*n_collect_children)(struct ivy_ast_node *, struct ivy_ast_node_iterator *);
+
 	size_t n_state_size;
 	size_t n_node_size;