Method push

publication/code/chapter04/infix_to_postfix.rs:19–22

(&mut self, val: T)

    }

    fn push(&mut self, val: T) {
        self.data.push(val);
        self.top += 1;
    }

    fn pop(&mut self) -> Option<T> {
        if 0 == self.top { return None; }