Split Lin package

[shym]

Refactor the Lin package into sub-libraries so that users can choose to use only the domain, the thread or the effect implementation. That way it is possible to test the domain and effect implementation in runs without running first the thread initialisation code.
Put the part of the interface that should not be used by external users into _internal modules. They can’t be really hidden (by using dune private_modules stanza for instance) without breaking the test suite.

This PR is really only about the couple of commits at the tip of that branch. They are currently based off an integration branch because it is meant to be merged when #112 will have been merged.

[jmid]

I've now taken a pass over this PR.
Overall I was very pleased to see how systematic it approaches the refactoring.
It made reading the diff commit-by-commit a pleasure! 😃

A few things I noticed - only related to naming really:

• Why change ApiSpec to Spec? I think the former is more descriptive. InterfaceSpec may be more in line with established OCaml terminology though...
• I'm not sure about the ExpandedSpec name as it is not very descriptive: such modules contain generators, printers, shrinkers, etc., so they are further from a 'specification'. I think of this old Lin as an internal test representation myself. Could Lin_internal.TestRepr and Lin_common.Make/Build_test_repr work? (other name suggestions welcome!)
• Finally the old "Lin_api" and "Lin" test name prefixes should probably be updated in a separate commit of the PR

[shym]

I rebased (well, largely rewritten) this PR on main.
I hope it is also a lot more thorough, thanks to #188: a few tests were not updated to the new interface.

I do agree with your naming remarks, so I did revert the changes I had made. I suggest we do rename (to InterfaceSpec and Make_test_repr for instance) in a further PR.
About the test name prefixes, I went with Lin DSL and left Lin intact. My reasoning was that the nuance is really only for us to know which test was really run, something like Lin internal would just add noise.

Last point that requires work (maybe in a further PR also?) is src/kcas that I left completely untouched since I don’t know whether that code is still alive (I should probably install that library to test changes I would do there if we want to update it).

[shym]

It also does not yet update the documentation, but I suggest we do this also in a separate PR. Since this one is modifying so many source files in our test suite, we would avoid a lot of conflicts by merging soon.

[shym]

And rebased on main.

[jmid]

Thanks a bunch for this 🙏
I went over the commits again, found a small nit, committed a fix, and merged into main