!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
So, changing this API to Source/Sink requires the new style IO? I guess, otherwise, you'd need to introduce a temporary glue layer, right?