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.
Remove Codec from versionInterfaces in meta (RESTMapper is now agnostic
to codec and serialization). Register api/latest.Codecs as the codec
factory and use latest.Codecs.LegacyCodec(version) as an equvialent to
the previous codec.
