Your array contains machine-sized Ints, which in practice are likely
either 32-bit or 64-bit, while a ByteString is the equivalent of an
array or 8-bit values. So you'll need to somehow convert the Ints to
Word8s. Do you know if you need big or little endian?

A basic approach would be:

* Use freeze to convert your IOArray into an IArray
* Use putIArrayOf and put (from cereal) to generate a Putter value
* Use runPut to generate a ByteString from that