class mac_final_triple :?iv:string -> ?pad:Cryptokit.Padding.scheme -> block_cipher -> block_cipher -> block_cipher ->Cryptokit.hash
c1, c2 and c3.  The input is run through c1 in CBC
        mode, as described for Cryptokit.Block.mac.  The final
        initialization vector is then super-enciphered by c2, then
        by c3, to provide the final MAC.  This construction results
        in a MAC that is as nearly as fast as Cryptokit.Block.mac
        c1, but more resistant against brute-force key search
        because of the additional final encryption through c2 and
        c3.