doFinal method
Store the digest of previously given data in buffer out starting at
offset outOff. This method returns the size of the digest.
Implementation
@override
int doFinal(Uint8List out, int outOff) {
  // FIPS 202 SHA3 https://github.com/PointyCastle/pointycastle/issues/128
  absorbBits(0x02, 2);
  squeeze(out, outOff, fixedOutputLength);
  reset();
  return digestSize;
}