Cardano Updates

I noticed the constructor was called `TestKey`, but `WithSerialised` is probably
nicer. In any case, it's a minor renaming that I thought would be
uncontroversial.

Outside of direct tests on the compact index, clashes are arguably not so
important: we can assume that the compact index handles clashes correctly
because we already test with clashes on the compact index directly.

This also fixes #580, restoring the generation of clashing keys.

If they're not unsliced, then tests will fail.

It was shrinking two values at the same time, which won't always lead to minimal
counterexamples.