Method right_subtree

code/chapter07/avl.rs:175–180

(&mut self)

    }

    fn right_subtree(&mut self) -> &mut Self {
        match *self {
            Null => panic!("Empty tree"),
            Tree(ref mut node) => &mut node.right,
        }
    }

    // 左右旋
    fn rotate_left(&mut self) {