!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 what about then Thingy takes the ops, then gives you an Iterator? Then you can have an "aggregate" op that just pulls out upstream ByteStrings until size limit is reached, then from that on it needs explicit pulls. So if you have small output (size limit not reached) then the iterator will have only one element, otherwise it will pull and output additional elements on iterator.next() calls