Port build to dune

[Leonidas-from-XIV]

This will make the build of the library a bit more future proof and easier for people to understand and contribute given dune is well-accepted.

[Leonidas-from-XIV]

I had to create calendar and calendarlib because dune doesn't like having public names not match OPAM names but I feel this is probably alright in this context.

[pmetzger]

@Leonidas-from-XIV Before I did in, did you also fix the weird not-releasing-from-trunk issue for this repo? We should really make it follow normal expectations.

[Leonidas-from-XIV]

No, I haven't looked at releasing yet, but I don't quite see what would prevent releasing from trunk? I tried this branch and it looks like when you install it you are getting the same API as the 2.04 release. It is a bit tricky to test but at least my surface-level testing worked, and so did the tests.

I also noticed that there is a 3.x branch with a dune port. Maybe the 3.x port can be made the default branch, to give it more visibility that work is progressing?

[pmetzger]

I recommend we simply pull the stuff from the branch onto the trunk and make this work the way every other thing works.

[Leonidas-from-XIV]

Just had a quick look at the changes in the 3.x branch and yeah, that's probably reasonable.

[pmetzger]

Please do it then.

[Leonidas-from-XIV]

As I don't have commit rights, I can only create a PR: #41

[pmetzger]

I've merged the PR to pull the 3.x branch to the main branch. If you could clean this up I'll merge after. Also, I can give you commit access if you would like.