Most normal codec use should perform defaulting. DirectCodecs should not
perform defaulting. Update the defaulting_test to fuzz the list of known
defaulters. Use the new versioning.NewDefaultingCodec() method.
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.
