!htc #15716: Akka HTTP with the new DSL

/cc @jrudolph @sirthias 

TODO:
 - [X] Port http-core
 - [x] Port rest of http
 - ~~Port to the new IO abstraction~~
 - [x] get rid of unnecessary materializer parameters
One thing that the `runWith` makes really clear (and to a lesser extent a method name that starts with `to`) is that this materializes the stream. I’d also find it nicer to just say `.headFuture`, but that is more ambiguous.