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.  
Are you sure this needs to take a Word8 as input. If it can take an Int, then you do not need fromIntegral in the function. Change it and see if anything breaks.