Extract akka-http-model, make akka-http-core depend on it

Akka HTTP's HTTP model is very much state of the art when it comes to a statically typed HTTP Model. This is confirmed by other projects, such as http4s, using the good ol' Spray model, and we know we improved a lot over the Spray model in Akka HTTP.

By making the model it's own module Play could (and should) use it for their typed headers feature (and later on entire model perhaps).
The model should include the parsing things as well.

It's ok that model depends on akka-streams, we like the entities being modeled as streams.

Discussed Play roadmap with @wsargent and this would be a great win for both of the projects we think.

Start by locating the existing HTTP model and parsing code, then inspect how akka-http-core and akka-streams are connected. The change is complete when the model and its parsing functionality form their own module and akka-http-core depends on it, while retaining the stream-based entities.

Written by the indexing model from the issue text.