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.

examples/examples_test.go

@@ -422,7 +422,7 @@ func TestExampleObjectSchemas(t *testing.T) {
 				if err != nil {
 					t.Errorf("Could not get codec for %s: %s", expectedType, err)
 				}
-				if err := codec.DecodeInto(data, expectedType); err != nil {
+				if err := runtime.DecodeInto(codec, data, expectedType); err != nil {
 					t.Errorf("%s did not decode correctly: %v\n%s", path, err, string(data))
 					return
 				}