Method postorder

publication/code/chapter08/avl.rs:379–388

(&self)

    }

    fn postorder(&self) {
        match self {
            Null => (),
            Tree(node) => {
                node.left.postorder();
                node.right.postorder();
                println!("key: {:?}", node.key);
            },
        }
    }

    fn levelorder(&self) {
        let size = self.size();