!str #15236 new transformer

This is far from finished, but I need to discuss.

The end goal of this is to provide a powerful, efficient, low level API for custom transformation operations. It is supposed to replace `Transformer`.

So far this is only using the API of the `impl.fusing.Op` (and subsclasses). These will be made more user friendly where needed. We can also consider providing higher level API that is more convenient for certain type of operations.

As first step I'm replacing all existing `Transformer` usage.

I'm currently stuck in `HttpEntity` and we must discuss how to go forward there. @jrudolph @sirthias should be part of that discussion.
@jrudolph Do I understand it correctly that these `transformDataBytes` are "only" used for encoding/decoding the bytes. Is Transformer a good interface for those? What do you think about creating a special purpose Transformer interface for them in akka-http-core, and be able to run that thing in a `DeterministicOp` when hooking in to a Flow.transform.