Method inorder

publication/code/chapter08/avl.rs:368–377

(&self)

    }

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

    fn postorder(&self) {
        match self {