Method enqueue

code/chapter03/hot_potato.rs:17–24

(&mut self, val: T)

    }

    fn enqueue(&mut self, val: T) -> Result<(), String> {
        if Self::size(&self) == self.cap {
            return Err("No space available".to_string());
        }
        self.data.insert(0, val);

        Ok(())
    }

    fn dequeue(&mut self) -> Option<T> {
        if self.size() > 0 {