!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.
This is a user error depending on the definition of the `transformDataBytes` function. In its current form its definition would be that the user is at error calling the function if the receiver is a Chunked entity that contains chunks with metadata.

We could also remove this constraint and just (silently?) drop the metadata but it's hard to say which is the less surprising behavior.