More CI jobs (and add a windows job)

[hhugo]

In addition to the existing CI, this PR adds 3 6 more CI jobs using the ocaml-setup github action.

• It gives more freedom regarding what version of ocaml we're testing against - this PR tests for 4.14 and 5.0
• It enables windows in the CI (in addition to Macos and Linux)
• It tests opam install as well

The down side is that it's slower than the existing jobs as it has to recompile the ocaml compiler everytime (modulo caches).
I kept the existing jobs as it gives faster feedback.

The second commit is necessary to have test pass on windows

The last commit 6ec3aa3 fixes the build on windows (following #135)

fix #98

[hhugo]

@dra27, I've followed your advice and used gitattributes instead of changing the tests

[hhugo]

I've added OCaml 5 to the CI (including windows) and fix the build on windows with 6ec3aa3 (as suggested by @dra27 )

[hhugo]

@xavierleroy, do you have any opinion on this ?

@dra27, can you check the CI script regarding how to use ocaml 5 on windows ?

[xavierleroy]

I'm a complete noob concerning Github actions, so thanks @hhugo for the CI script you proposed. I was able to fix a typo in the script, but that's about the extent of my competence here.

The script runs in about 22 minutes, which is a lot of CPU cycles for such a small library and a small test suite. On the other hand, there is not much activity on Zarith, so the script will not run very often.

Let's merge this PR and we'll see how it works in practice.

[hhugo]

@xavierleroy, FYI, the CI tooks ~10minutes on your latest commit, probably thanks to caching.
It is still significantly longer than the original CI job but already much better than the 22minutes you've witnessed.