base58.hs haskoiner reading

Basically, now the en(de)coding to base58 is done by readInt and showIntAtBase from Numeric. Im not sure if its faster but is easier to read. Moreover I created the function decodeBase58I to be consistent with having all the encoding functions and its inverse.  
If b58 returns Maybe Int as I suggested, you do not need the first call to fromIntegral.
If b58 takes an Int as I suggested, you do not need the second call to fromIntegral.