Migration to v0.7

As can be seen there are several rough edges. More boilerplate for simple flows, e.g. the foreach-oncomplete.

The tutorial documentation is not updated yet.
Well, we could always add that back on, returning a `Future[Unit]`, but only if the pain really is big enough. We could also offer such things (like toPublisher as well) as extension methods that need to be imported. Another possibility would be to interject a marker, e.g.
~~~scala
FlowFrom(text.toLines).map(_.toUpperCase).terminal.foreach(println)
~~~
which would allow us to keep the FlowOps clean from terminal operations but still offer them in a consistent fashion that also works for Java.