Prototype of flexible merge

Still only a prototype, but would be great with a round of feedback for tomorrow's work.

I tried the "return next behavior" approach (see savepoint1 if you are interested), but that was a dead end. We want to allow return of behavior with new input and output types, e.g. Zip, and I couldn't get that to work without explosion of types. Reverted back to become.

// cc @drewhk @sirthias 
I will not rename those now. Depleted is used at other places, confusing or not, I have to learn it.
Anyway, if I understand you correctly the completionHandling.onCompleted should be called only when depleted(id) is changed to true.