!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
Nice!
Could we even make something like

    Source(Chunk(r.get) :: Nil).concat(entityChunks)

work?