Method push

publication/code/chapter04/linked_list.rs:40–47

新节点总是加到头部，

(&mut self, elem: T)

    // 新节点总是加到头部，
    fn push(&mut self, elem: T) {
        let node = Box::new(Node {
            elem: elem,
            next: self.head.take(),
        });
        self.head = Some(node);
        self.size += 1;
    }

    // take 会取出数据留下空位
    fn pop(&mut self) -> Option<T> {