Add Data.ByteString.Char8.to{Low,Upp}er

Case conversions can benefit from low-level tricks. While `Data.ByteString.Char8.map Data.Char.toLower` converts byte by byte, one can actually read 8 bytes at once as `Word64` (or even more, when vectorised instructions are available), build a mask checking which bytes are uppercase (this can be done in a single instruction), then shift the mask by 5 and add to the original `Word64`.