Split up Convenience into separate classes

[Simn]

I always forget why this actually works, but it does. The nice thing about this is that we can use names like isCancellationRequested for multiple things.

Also deals with the weird checkCancellation in UnboundedWriter.

[Simn]

Turns out C++ has issues without #106 too, and not all of them are about #85 either. I didn't notice this before, so I wonder if it's from some more recent change. We gotta sort this out...