Use opam-based CI
I'd like to use the same opam-based CI setup that we also use for lambdaRust. This will also make it trivial for us to run the CI with both Coq 8.5 and 8.6, and to change Coq versions in the future (no changes in the CI image needed for this any more, yay :D ).
However, to make that work, we have to stop importing
_CoqProject. So like in lambdaRust, I'd like to move everything into a subfolder
This will break all merge requests, so we should get at least some of them in. !30 (merged) will hang around for a while, so whatever. But I'd like to wait until we get at least !25 (merged) or !22 (merged) in.