operator * method
Multiplies this BigRational by the given BigRational and returns the result.
other The BigRational to be multiplied by.
returns A new BigRational representing the product of this BigRational and the given BigRational.
Implementation
BigRational operator *(BigRational other) {
  final BigInt resultNumerator = numerator * other.numerator;
  final BigInt resultDenominator = denominator * other.denominator;
  return _reduce(resultNumerator, resultDenominator);
}