+str #15079 Add 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 
Yes, this behavior is buggy: terminating a stream should not interfere with the ordering it shall normally guarantee. But you can open a ticket for fixing it later if that does not fit within this sprint.