Method postorder

publication/code/chapter08/binary_tree.rs:228–238

(&self)

    }

    fn postorder(&self) {
        match &self.left {
            Some(node) => node.postorder(),
            None => (),
        }
        match &self.right {
            Some(node) => node.postorder(),
            None => (),
        }
        println!("key: {:?}", &self.key);
    }

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