!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.
So, there is one idea I can sketch out. We have a Thingy[T, U] which has a series of ops. When you call thingy.run() then it gives you a result Either[U, Source[U]]. Basically depending whether some iteration or size limit has been met or not you get a strict result, or an asynchronous stream that continues from the point where the limit has been reached after emitting the accumulated result so far. (the types here are just pseudo-code, feel free to propose something else)