Provide `Lwt.sleep` as a Unix-less function about lwt HOT 3 OPEN

Hi,

Your approach seems a bit invasive for our use case (MirageOS) where we implement our own scheduler with our own time function (which depends on the backend). Currently, you can see an example of our loop here. I don't have, a priori, remarks against that but we must be in sync about your plan and our current ecosystem to be sure that we still able to inject our time function and our "scheduler" regardless what you want to do.

More concretely, we must be sure that Lwt.sleep can be easily replaced by our specific syscall. Then, we will continue to implement our run function as we do currently (may be @hannesm/@avsm will be more advised than me about that).

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Thanks for the comment. The timing is what is giving me the most doubts and I'm happy to have a more concrete example to check. I'll have a look at it and see if I can refine the proposal to take it into account.

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What's the actual problem you're trying to fix here, @raphael-proust @hhugo? It would be good to see a small example of something that needs to sleep in a platform-independent way for both jsoo and unix compilation, and then it'll be easier to figure out how Mirage fits into it. In most cases, the amount of time something sleeps for is pretty closely tied to the IO usage, which in turn is tied to the backend in use.

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