Prepare a release on opam

[thibautbenjamin]

Catt should be available as a package on opam, and catt-coq-plugin should be available as a coq plugin on opam as well.

This requires fixing the dune-package file to write the correct dependencies. Notably, the dependency for Coq must be v.8.20, as the API keeps changing

Issues to fix and pull requests to merge before the release:

• Kernel Performance Improvements #56
• Make the opam dependency clear #68
• update the README file #65
• Better handling errors in conditional meta-operations #67
• Fix error in name of iterated functorialisation #66
• Feature/better contexts #71
• use space separation for lists #77