!htc #15726: 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
I think it is hard to see which parts belong together in this infix notation chain, perhaps add parens:

    (Source(Chunk(r.get) :: Nil) ++ entityChunks) :: Nil

or

    List(Source(List(Chunk(r.get))) ++ entityChunks)