update method
Add len bytes of data contained in inp, starting at position inpOff
ti the digested input.
Implementation
@override
void update(Uint8List inp, int inpOff, int len) {
  int nbytes;
  nbytes = _processUntilNextWord(inp, inpOff, len);
  inpOff += nbytes;
  len -= nbytes;
  nbytes = _processWholeWords(inp, inpOff, len);
  inpOff += nbytes;
  len -= nbytes;
  _processBytes(inp, inpOff, len);
}