As noted in #52, there are some events on Windows that can only be detected by calling one of the WaitFor family of functions (WaitForSingleObject, WaitForSingleObjectEx, WaitForMultipleObjects, etc. etc.). Some examples include child process exit (required for #4), and detecting console input (required for #174).

To resolve this issue, we should provide a trio.hazmat.WaitForSingleObject function that takes a HANDLE, and waits for it, and is cancellable.

The most straightforward way to do this is to allocate a thread to sit in WaitForSingleObjectEx, which can do an "alertable wait", which means that it's possible to cause the function to wake up early by calling QueueUserAPC. So we would use QueueUserAPC from inside our abort callback to implement cancellation. There are some more notes in _io_windows.py.