!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 tried to use FlowOps and higher kinds, but failed. This does not mean that it is impossible, just that I am not good enough with types. Help is welcome.