Native support for Windows

[Halbaroth]

After #584, we stop building and testing Alt-Ergo on Windows with Cygwin. It seems that we have users on Windows. We should build and test AE on Windows again.

[Halbaroth]

Besides, I hope Dolmen works on Windows. I think @Gbury did the job to make it works ;)

[Gbury]

Yes, dolmen works on windows ! I've worked hard to ensure that (and I have CI to check for that as well).

[Gbury]

Small caveat: on windows, time limits are slightly less reliable since they use the GC alarm mechanism instead of Unix timers, but I don't think it's a problem.

[bclement-ocp]

IIRC the problem before #584 was not that Dolmen did not work for Windows but that the version we used was not available on the Windows OPAM repository ( https://github.com/ocaml-opam/opam-repository-mingw ) which is lagging behind the "main" OPAM repository. Currently we use a pinned version so that shouldn't be a problem to reinstate the build; if/when we make a release, we will need to take care of this.

(Note: if my understanding of this is correct, opam-repository-mingw should no longer be necessary once OPAM 2.2 is release, which would make the problem go away — in fact I do not understand if it is currently required or not)

[bclement-ocp]

Additional note: interested users clarified that they wanted native Windows support, not Cygwin. AFAIK that means either waiting for OPAM 2.2 or using Diskuv. I'd say we wait for OPAM 2.2 as there is no pressing need.