Core testsuite workflow

[abbysmal]

This PR aims to implement a workflow to run Core's testsuite once a day.

The long run ideal would be to include in a matrix other pieces of Core's ecosystem (async_unix, core_kernel, to name a few) and to keep tab on those from within our CI environment, since running their testsuite is rather hard.

This follows @emillon's approach in #624 (thanks!), and the monorepo is a fork of his, hosted at ocaml-multicore/core.

This PR and monorepo will need updates once our rebase to 4.13 is done, and I will try to dig further into integrated more libraries in this run, when possible.

As such I will mark it as a draft for now.

Note that in Core's case, I included a fix for the channel layout assumption in bigstring, since this will be a required upstream change in the long run.

[kayceesrk]

Which is the branch used in ocaml-multicore/core? Is is monorepo-0.14? How does the workflow file pick this branch? Should we make monorepo-0.14 the default branch on ocaml-multicore/core because that's the relevant one.

[abbysmal]

I think we can make the default branch monorepo-0.14 indeed, though the workflow files picks it up as a part of the checkout actions, like this:

        with:
          repository: 'ocaml-multicore/core'
          ref: 'monorepo-0.14'

[emillon]

If the repository contains core.opam.locked, it's better to omit this line, that way you'll get reproducible results. And you can update the dependencies by hand by pushing a new lockfile (obtained by locally running opam monorepo lock).

[abbysmal]

Right, anyway I noticed it fails nowadays and cannot recreate a lock file (because of some ocaml-base-compiler conflict at some point, I'm not sure I understand it). So it leaves the previous lockfile in place, apparently.
I will remove that line, thanks.