!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 +1 to that. 

@drewhk We are in deep optimization territory there already, so I'm not sure if we'd really need what you suggest as an actual API. For me, it would be enough if you could limit the number of steps the iterator runs and provide the fallback manually. For this particular use-case it would be even better if we could manually pull from the output side (i.e. just do single steps) synchronously, so that we could stop once too much data had been aggregated. Does that make sense?