Frenetic-related releases

This pull request includes the following libraries and versions, in preparation for a Frenetic release to come shortly:

* packet.0.2.1 ([release notes](https://github.com/frenetic-lang/ocaml-packet/releases/tag/v0.2.1))
* topology.0.1.0 ([release notes](https://github.com/frenetic-lang/ocaml-topology/releases/tag/v0.1.0))
* openflow.0.3.0 ([release notes](https://github.com/frenetic-lang/ocaml-openflow/releases/tag/v0.3.0))