[Rust] Implementing Display trait for Rational

impl fmt::Display for Rational {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let is_positive = (self.num > 0 && self.denom > 0)
            || (self.num < 0 && self.denom < 0);

        let sign = if is_positive { "" } else { "-" };

        write!(f, "{}{}/{}", sign, self.num.abs(), self.denom.abs())
    }
}