idxset: Add some comparison, reverse iteration, and binary operations

I implemented some idxset operations expecting to use them to generate conflicting device groups. I ended up only needing isdisjoint(devices, dev->conflicting_devices) and issubset(devices, dev->supported_devices) to check conformance, and pa_idxset_steal_last(devices) to do some sort of backtracking. Since I've already implemented them, I've put all of them to this MR in case you think they'd be useful.