Expect tests for generated code in examples/

[mbarbin]

Hello team,

Following our discussion in issue #204 , I've made a small adjustment to the testing workflow. This is a minor change, but I believe it could reveal beneficial.

Here's what's been done:

1. Expect Tests: For each ml and mli file generated by a dune rule in the src/examples/ directory, corresponding *.ml.expected and *.mli.expected files have been added. This will help us keep track of the impact of PRs on the generated code.

2. Dune Rule: A dune rule has been added to diff the generated files against their expected reference as part of dune @runtest. In case the generated code changes, updating the expect files is straightforward. You just need to run dune promote, and the expect files will be updated by dune to match the new output.

This change doesn't introduce any new dependencies or affect the non-testing code. It's a small step towards continuing to improve on the existing testing workflow.

I find that it can be hard to know beforehand whether this new test setup will add undue friction. If in practice, this change doesn't work as well as I hope, it can easily be reverted without impacting the rest of the code.

Looking forward to your feedback.

[c-cube]

Thank you, that makes sense!