invert method

Implementation

@override
VestaFq? invert() {
  if (isZero()) return null;
  final tmp = powVarTime([
    BigInt.parse("0x8c46eb20ffffffff"),
    BigInt.parse("0x224698fc0994a8dd"),
    BigInt.parse("0x0"),
    BigInt.parse("0x4000000000000000"),
  ]);
  return tmp;
}