!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.
@patriknw I understand that from a theoretical viewpoint but it seems strange that these variables live in this class but can neither be read nor written by implementors of this class. If the implementation of this class cannot access these variables, what else can (and by which means)? E.g. is it possible to write a component that wraps another stage with some sort of general behavior so that these variables will still be set in the right way?