!htc #15726: Akka HTTP with the new DSL

/cc @jrudolph @sirthias 

TODO:
 - [X] Port http-core
 - [ ] Port rest of http
 - [ ] Port to the new IO abstraction
 - [ ] get rid of unnecessary materializer parameters
To be honest, I never really liked the `toFuture` name either.
But I very much do like being able to use a concise "fluid" notation as much as possible (So `sourceA concat sourceB` rather than `Source.concact(sourceA, sourceB)`.)
I'd vote for `headFuture` in analogy to `headOption` that is defined on collections.