! routing: CompletionMagnet: gone, streamlining completion API: accomplished

This change comes from the insight that with the new marshalling
infrastructure in place CompletionMagnet doesn't serve any higher
purpose.

By removing the now useless magnet layer composition is reinstantiated and
unification as shown in the accompanying test works (with a little help
from type annotations)
= httpx: code reorganization to DRY up implementation of new ToResponseMarshallers
Merge pull request #597 from jrudolph/w/streamline-future-marshalling

Streamline completing by replacing CompletionMagnet in favor of the more generic ToResponseMarshallable