code: func (d *digest) Sum(in []byte) []byte {
  	s := d.Sum64()
  	in = append(in, byte(s))
  	in = append(in, byte(s>>8))
  	in = append(in, byte(s>>16))
  	in = append(in, byte(s>>24))
  	in = append(in, byte(s>>32))
  	in = append(in, byte(s>>40))
  	in = append(in, byte(s>>48))
  	in = append(in, byte(s>>56))
  	return in
  }
docs: // Encode in little endian