Prep work for Clap

Description / Motivation

This was extracted from !25 (merged). This is prep work that does not depend on Clap. In particular, it adds some tests.

The third commit (e4f77c27) is actually from !46 (merged) — it turns out I had done the same patch both in !25 (merged) and !46 (merged), except that the commit was not split in !25 (merged).

How to Test the MR Manually

Rely on the CI

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• Add tests in test/, if relevant.