=str #15755 #15756 source and sink

sorry, deleted the old branch by accident, previous discussion was at #15786 
But why not Sink[.., Unit] then? I find the Any highly confusing, just imagine someone trying to write a Sink, for example one that takes an Rx pipeline and attaches to the end of the stream. What should I return, since the type is Any? With Unit it is clear that I should not return anything.