Update and expand quickcheck subpackage

This pull request makes the following changes to the quickcheck subpackage:

* rename the `Packet_Arbitrary` module to `Arbitrary_Packet`;
* include an interface file for `Arbitrary_Packet`; and
* move `Arbitrary_Base` module from [frenetic-lang/ocaml-openflow](https://github.com/frenetic-lang/ocaml-openflow) to this project.