!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.
RangeDirectives also use a similar method. We had a specialized version, before, taking two functions looking like this: 

```scala
def mapDataBytes(f: ByteString => ByteString, trailer: () => ByteString): HttpEntity
```
but this was already almost a transformer and as this method is definitely thought to be used by any consumer of the API it makes sense to keep it as general as possible. What would a special-purpose Transformer buy us?