!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.
Maybe I give a code example how a simple synchronous execution can look like

    val interpreter = new OneBoundedInterpreter(upstreamByteString +: ops :+ downstreamByteString)
    interpreter.init()
    downstream.kickOff()
    val outBS = downstream.readOut()

This is all synchronous if the upstream/downstream ops do everything synchronously. If you want, you can put the ops in a Flow and have an asynchronous stream, so they are reusable in that regard. There is no support for stream-of-stream stuff, this is at this stage only replacing Transformer. What is the functionality missing compared to Transformer? We might be able to add that.