qc: Move helper functions to Arbitrary_Base

Arbitrary_Base actually comes from frenetic-lang/ocaml-openflow. Pushing
it down here to eliminate redundancy.
qc: Rename Packet_Arbitrary to Arbitrary_Packet
qc: Add .mli, additional arbitrary packet field instances
Merge pull request #31 from frenetic-lang/qc

Update and expand quickcheck subpackage