Method append

code/chapter03/lvec.rs:70–76

栈末尾加入数据

(&mut self, other: &mut Self)

    // 栈末尾加入数据
    fn append(&mut self, other: &mut Self) {
        while let Some(node) = other.head.as_mut().take() {
            self.push(node.elem);
            other.head = node.next.take();
        }
        other.clear();
    }

    fn insert(&mut self, mut index: usize, elem: T) {
        if index >= self.size { index = self.size; }