Method add_rear

code/chapter03/deque.rs:29–36

Vec 首部为队尾

(&mut self, val: T)

    // Vec 首部为队尾
    fn add_rear(&mut self, val: T) -> Result<(), String> {
        if self.size() == self.cap {
            return Err("No space avaliabl".to_string());
        }
        self.data.insert(0, val);

        Ok(())
    }

    // 从队首移除数据
    fn remove_front(&mut self) -> Option<T> {