invert method

Implementation

@override
PallasFp? invert() {
  if (isZero()) return null;
  final tmp = pow([
    BigInt.parse("0x992d30ecffffffff"),
    BigInt.parse("0x224698fc094cf91b"),
    BigInt.zero,
    BigInt.parse("0x4000000000000000"),
  ]);
  return tmp;
}