Remove _CoqProject by default

Fixes #16 (closed). make coqproject generates a sensible default _CoqProject for the core implementation and case_studies that can be extended locally as needed. It doesn't include stdlib, as the custom theories for that make it hard to guess what we need to include. But anyone working on the stdlib will probably have the knowledge to add it themselves, if their editor doesn't support dune.

enabled an automatic merge when the pipeline for 09826842 succeeds

merged