Implement Skipjack block function

[Bender250]

Implement Skipjack - old NSA block cipher.

This is "good first issue" - for new contributors preferably, as the implementation should be exemplary for CryptoStreams workflow.

Steps:

1. Find no licence/MIT/BSD licensed implementation, preferably reference version (this is more difficult here, as Skipjack was classified crypto). Do not use heavilly optimized implementations, it will be hard to use them correctly.
2. Implement it to CryptoStreams, in appropriate folder, with appropriate naming - inspire yourself by AES or others.
3. Write test vectors.
4. Round reduce the function.
5. Document it in https://github.com/crocs-muni/CryptoStreams/wiki/Block-ciphers-list

[Bender250]

(We can duplicate this function with A5/1, A5/2, Streebog).

[Bender250]

KeeLoq