Add friendly message to Chk for Reffer != nil

Now you get told you're holding it wrong when you fail
to import the enc package and get the global definition for
Reffer.

types/cached_ref.go

@@ -9,7 +9,7 @@ type cachedRef ref.Ref
 
 func (cr *cachedRef) Ref(v Value) ref.Ref {
 	if ref.Ref(*cr) == (ref.Ref{}) {
-		Chk.NotNil(Reffer)
+		Chk.NotNil(Reffer, "Reffer is nil; you probably need to import 'enc'.")
 		*cr = cachedRef(Reffer(v))
 	}
 	return ref.Ref(*cr)