XSalsa20 implementation

XSalsa20 implementation, base on the existing Salsa20 engine with a couple of tweaks to allow the key setup and nonce size to vary

XSalsa20 is a version of the Salsa20 stream cipher with an extended (192 vs 64 bit) nonce.

Test vectors are copied from the [cryptopp implementation](https://github.com/murrificus/cryptopp/blob/master/src/TestVectors/salsa.txt), which were generated using the nacl XSalsa20. There don't appear to be any official test vectors.