Maintain an executable job in the repo to keep off dead links

[Sudha247]

Elsewhere, @avsm suggested adding some sort of a CI job that can be executed here in the repo so as to avoid stale and outdated links. I think this is a great idea to keep this repo up to date. There are several ways to do this -

• Add a .opam file for unreleased Eio ports and multicore libraries. Eventually we want them all to be in opam-repository and the intention is not to make this an alpha-repository of sorts. Having all the opam files in a single place could help with checking their builds with a single opam pin command.
• Have an updated monorepo. The ones that exist are https://github.com/avsm/eeww and https://github.com/TheLortex/networking-experiments, and