Method add_front

code/chapter03/deque.rs:19–26

Vec 末尾为队首

(&mut self, val: T)

    // Vec 末尾为队首
    fn add_front(&mut self, val: T) -> Result<(), String> {
        if self.size() == self.cap {
            return Err("No space avaliable".to_string());
        }
        self.data.push(val);

        Ok(())
    }

    // Vec 首部为队尾
    fn add_rear(&mut self, val: T) -> Result<(), String> {