packet v0.3.0 and openflow v0.4.0 release

Release notes can be found [here for packet.0.3.0](https://github.com/frenetic-lang/ocaml-packet/releases/tag/v0.3.0) and [here for openflow.0.4.0](https://github.com/frenetic-lang/ocaml-openflow/releases/tag/v0.4.0).

Related to resolving ocaml/opam-repository#2140.