Feistal network -> Feistel network

    (h/t Marsh Ray @marshray for this idea)

    This is a randomization/reshuffling function based on a crypto

    "Feistal network" as describ ed in the paper:

    "Feistel network" as describ ed in the paper:

    'Ciphers with Arbitrary Finite Domains' 

        by John Black and Phillip Rogaway 

    The cryptographic strength of this construction depends upon the 

    number of rounds, and the exact nature of the inner "F()" function.

    Because it's a Feistal network, that "F()" function can be almost

    Because it's a Feistel network, that "F()" function can be almost

    anything.

    We don't care about cryptographic strength, just speed, so we are