Add QuickCheck test for Kleene idempotence axiom.

I was running into some trouble because I wasn't sure if there was a way to convert a `Packet.packet` to a `NetKAT_Types.packet`. I'm not sure if there's some fundamental difference between the two. I just really wanted to see something work so I hacked up `NetKAT_Arbitrary.arbitrary_packet` to generate a `NetKAT_Types.packet`. This test currently passes, although it isn't very good because it only tests packets with empty headers and switch ID 0.

If you think it would be simple enough to try and implement `arbitrary_packet` for real, I could try to do that.