!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.
I agree, the simplest API would just be:

```scala
def transformDataBytes(flow: Flow[ByteString, ByteString]): HttpEntity
```

The main reason it currently isn't is that using a `Transformer` we can strictly map over `HttpEntity.Strict` using a transformer. However, I guess you could see this already as an optimization that should actually be done in the stream infrastructure so playing with the alternatives certainly makes sense.