Experimental work to add cohttp support for opam-cross-windows

[Jo-Blade]

This is a work in progress to support cohttp (and especially cohttp-lwt-unix) in opam-cross-windows. My initial goal was to try to cross-compile a custom app that use cohttp-lwt-unix but it's a partial success only.

I managed to build and make cohttp work only with plain old HTTP and not HTTPS. For that I had to package a lot of stuff, feel free to take a look and give me some feedback.

I couldn't make ocaml-ssl detect windows ca-certificates. As an alternative I've considered ocaml-tls but couldn't cross compile it at all because of "make inconsistent assumptions over implementation".

The commits tree is very ugly due to a lot of trials and errors but if people are interested I can try to clean up a bit. For some packages (ptime and mtime) I had to modify the source code to convert the project to dune to be able to compile. You can find the patches in the "mnt" folder to have the same sources as me but it need to be properly integrated to opam when I will have more time.

I decided to create a draft pull request to help future people to learn from my try, even if it's not fully functional. If people are interested to help me I'm still interested in giving cohttp support otherwise I'm not sure I will continue cause of lack of time.

Thank you for your project