Add Windows support

[luser]

Windows provides an API to associate an IO completion port with a Job object, by calling SetInformationJobObject with JobObjectAssociateCompletionPortInformation and passing in a JOBOBJECT_ASSOCIATE_COMPLETION_PORT.

The MSDN docs say the notifications aren't reliable, but don't explain why and I can't find any evidence to support that. I think it's worth trying it out, in any event.

[luser]

I think we'd want to get this mio issue fixed before attempting this: tokio-rs/mio#526 .