!str #15950 Add runWith and remove toX 

This should cover most things and is ready for review, but I will check if I have covered everything on monday (and add some docs).

* runWith(drainWithKey) and runWith(tapWithKey) returning the
  materialized drain/tap
* remove toPublisher, toSubscriber, and friends, in favor of using
  runWith
* MaterializedMap is the return type for both flow and graph
I think this is too verbose. The convenience method reads much nicer.