coro: redesign

[Cloudef]

I don't like the current coro api, it's too high level and the scheduler is not a good abstraction.

Ideally coro example should look something like this:

while (!client.completed or !server.completed) {
    _ = client.schedule(&io); // returns either .yielded or .complete
    _ = server.schedule(&io);
    io.schedule(null); // null == optional timeout
}

This makes application completely in control of the scheduling, and the task / frame needs less state internally. The fundamentals should be flexible and solid enough to open doors for higher level abstractions.