Added xorish operators to Data.Bits.Lens

Inspired by http://en.wikipedia.org/wiki/Rijndael_S-box#Implementations

```haskell
affineX :: Word8 -> Word8 -> Word8
affineX c b = b &~ forM_ [0..7] twiddle
  where
    twiddle j = do bitAt j   .= getBit b 4
                   bitAt j .^.= getBit b 5
                   bitAt j .^.= getBit b 6
                   bitAt j .^.= getBit b 7
                   bitAt j .^.= getBit c 0
                   where
                   getBit n o = n ^. bitAt (mod (o + j) 8)
```