Migrate streams to reactive-streams 0.4.M1

So that produceTo is not the same as this produceTo :) The one which is changed is because that method existed only on Producer, which behind the scenes just called subscribe. OTOH the other produceTo is a materializing step on Flow/Duct. I was thinking renaming that, too, but was not completely sure that calling it subscribe is the right thing. I will open a ticket and we can discuss there what it should be named,