!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 think the main thing (which we could also need elsewhere) is the ability to emit a stream of results without having to lift the whole transformer to return a stream of streams and need to `concatAll` afterwards. But this again wouldn't be safe to do here synchronously for the same reason it isn't safe in the current version: with highly compressed data you can run a DOS attack using the deflater if it decompresses arbitrarily big ByteStrings (which it currently does). So, because of this problem I think the previous interface (using just Transformer) is broken and needs to be replaced with something that allows to contain `concatAll` and is executed asynchronously.