Is there a way to make operations cancelable?

Explore some way to combine/bring in CAF (cancelable async functions) capabilities so that fasy's async iterations are optionally cancelable.

This might need to be a separate namespace where the API methods assume the cancelable token being passed in, or it may just be that existing methods could be polymorphic in some way.

Also to be explored: should fasy re-implement the cancelation, or should there be an adapter that can be optionally applied which delegates to CAF (which would need to be present separately) when needed.