!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.
E.g. for highly compressed data the `Deflate.Decompressor` may currently produce huge `ByteString` that may even blow up the heap (I need to check this and create a ticket). With the proposed API above we could implement it differently so that we could stream the output instead and present the whole transformation as a `Flow[ByteString, ByteString]`.