Simplify Codec and split responsibilities

Break Codec into two general purpose interfaces, Encoder and Decoder,
and move parameter codec responsibilities to ParameterCodec.

Make unversioned types explicit when registering - these types go
through conversion without modification.

Switch to use "__internal" instead of "" to represent the internal
version. Future commits will also add group defaulting (so that "" is
expanded internally into a known group version, and only cleared during
set).

For embedded types like runtime.Object -> runtime.RawExtension, put the
responsibility on the caller of Decode/Encode to handle transformation
into destination serialization. Future commits will expand RawExtension
and Unknown to accept a content encoding as well as bytes.

Make Unknown a bit more powerful and use it to carry unrecognized types.

pkg/runtime/error.go

@@ -37,3 +37,11 @@ func IsMissingKind(err error) bool {
 func IsMissingVersion(err error) bool {
 	return conversion.IsMissingVersion(err)
 }
+
+func NewMissingKindErr(data string) error {
+	return conversion.NewMissingKindErr(data)
+}
+
+func NewMissingVersionErr(data string) error {
+	return conversion.NewMissingVersionErr(data)
+}