Helper functions for implementing BlockCipher for Blowfish.

I'm updating Stephen's code related to issue #23.  His work-around for using a different key size for Blowfish was broken by your removing the BlockCipher interface altogether.  I tried making my own BlockCipher instance that uses only functions you export, but the result was that automated tests took 5 times as long to complete. If you accept this patch, then I can use this instance which runs much faster:

```haskell
instance BlockCipher Blowfish128 where
	blockSize = Tagged 64
	encryptBlock (Blowfish128 b) = Blowfish.encryptBlock b
	decryptBlock (Blowfish128 b) = Blowfish.decryptBlock b
	buildKey                     = fmap Blowfish128 . Blowfish.buildKey 
	keyLength = Tagged 128
```