(Only for feedback) +str #15977 Rename and change types in the new DSL

There are still a lot of TODOs here (especcially documentation), but I would like som feedback on what has been done.

* Clean up type signatures
* Chang names of Source/Sink/ProcessorFlow to Faucet/Drain/Pipe
* Introduce new Source/Sink/Flow
* Make all Pipe types package private
* Change the TypeGraph DSL to work on Flow/Sink/Source
I suggest you should keep Drain and Tap public in this PR, and we can consider them more later.
I suspect that the new `runWithSink` will probably have to be `runWithDrain` (I have started prototyping that).