Convert single GV and lists of GVs into an interface that can handle
more complex scenarios (everything internal, nothing supported). Pass
the interface down into conversion.
Add a recognizer that is capable of sniffing content type from data by
asking each serializer to try to decode - this is for a "universal
decoder/deserializer" which can be used by client logic.
Add codec factory, which provides the core primitives for content type
negotiation. Codec factories depend only on schemes, serializers, and
groupversion pairs.
