Use the derived instance for Ord and add a test case.

There was a bug in my implementation of Ord for Octets that was
pointed out by Michael McKibben. As a result, the octets corresponding
to (for example) 2 and 4 would compare incorrectly. This is fixed by
simply adopting the derived Ord instance for Octets. Why didn't I do
that in the first place?

With the bug fixed, I've added a QuickCheck property to ensure that
the Ord instances for Octet and Int agree when the Int is between 0
and 255.