Add support for anonymous pipes

[notgull]

It would be useful if this crate supported Windows anonymous pipes.

I probably can't make any progress on this until #76 is complete, as this would require the backend to not be wepoll.

[kchibisov]

Given that your patch merged, how far are we from anon pipes? Asking since rust time bombed mio 0.6.x (it has future incompatible code) and we must find an alternative.

[notgull]

We still need to support direct OVERLAPPED operations in our use case. However, looking at your use case, it looks like you use [mio-anonymous-pipes], which "cheats" by putting the pipe onto a thread pool. Therefore, you might be able to get away with just wrapping the pipe in an Unblock and using it like that.

[notgull]

Not as much of a priority anymore as we've worked around this limitation in alacritty. This is mostly a duplicate of #97 now, as we would need overlapped operation support to truly support anonymous pipes in polling.