Hide common codec methods under helpers

The pending codec -> conversion split changes the signature of
Encode and Decode to be more complicated. Create a stub helper
with the exact semantics of today and do the simple mechanical
refactor here to reduce the cost of that change.

pkg/api/helpers.go

@@ -153,7 +153,7 @@ func AddToNodeAddresses(addresses *[]NodeAddress, addAddresses ...NodeAddress) {
 }
 
 func HashObject(obj runtime.Object, codec runtime.Codec) (string, error) {
-	data, err := codec.Encode(obj)
+	data, err := runtime.Encode(codec, obj)
 	if err != nil {
 		return "", err
 	}