!htc #15716: 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
It is not clear to me how `withContext` could be implemented as generically as the signature suggests: there is no 1-to-1 correspondence expressed between inputs and outputs, so using a bypass graph would need domain specific knowledge, and transforming all processing steps to funnel through the context seems tedious at best (i.e. not sure whether it can really be done, or if it can whether that would be very inefficient). Do you have more information on how this could work?