ThreefishEngine
Implementation of the Threefish tweakable large block cipher in 256, 512 and 1024 bit block
            sizes.
            
                
1024 bit block size - Threefish-1024
            
256 bit block size - Threefish-256
            
512 bit block size - Threefish-512
            
Constructs a new Threefish cipher, with a specified block size.
            
Initialise the engine.