Add Windows IOCP-based I/O executor

[lewissbaker]

Implement a simple single-threaded event loop that uses Win32 IOCP to dispatch async I/O completion events so that we can build and run examples that do file I/O on the Windows platform.

We can borrow some of the implementation details from cppcoro's io_service class.

[Mrkol]

How does this relate to #66?
Also, any chance of this getting some love in the nearest future? My hope is that one day unifex will have one single io_context like asio...