Method postorder

code/chapter07/binary_tree.rs:80–84

(&self)

    }

    fn postorder(&self) {
        if !self.left.is_none() { self.left.as_ref().unwrap().postorder(); }
        if !self.right.is_none() { self.right.as_ref().unwrap().postorder(); }
        println!("kes is {:?}", &self.key);
    }
}

// 前中后序遍历: 外部实现