Windows support

[CatherineGasnier]

Who here uses hh on Windows? Does it work? I see we have a bunch of code supporting Windows but part of it is fairly complicated (see e.g. timout.ml), and I'm very tempted to clean up that module (would help a fix I'm trying to make). We never test it internally, so we don't even know if it works. Let me know if you're opposed to me doing that clean up