Release with eqaf-cstruct being a separate package

[hannesm]

Hey,

it would be great if both #45 and #43 could be merged, and a release could be cut. This is a breaking change, but the advantage would be that our release train for mirage-crypto not using Cstruct.t could start with this package. Certainly, this is not strictly required, but it just feels much better if I can really test the universe where cstruct is just not installed. :)

Thanks a lot!

[hannesm]

awesome, done in #47 and ocaml/opam-repository#26094