lang: core, funcs: Port some functions to CallableFunc API

Some modern features of our function engine and language might require
this new API, so port what we can and figure out the rest later.

lang/funcs/structs/channel_based_sink.go

@@ -108,15 +108,20 @@ func (obj *ChannelBasedSinkFunc) Stream(ctx context.Context) error {
 				return nil // can't output any more
 			}
 
-			value, exists := input.Struct()[obj.EdgeName]
-			if !exists {
-				return fmt.Errorf("programming error, can't find edge")
+			args, err := interfaces.StructToCallableArgs(input) // []types.Value, error)
+			if err != nil {
+				return err
 			}
 
-			if obj.last != nil && value.Cmp(obj.last) == nil {
+			result, err := obj.Call(ctx, args) // get the value...
+			if err != nil {
+				return err
+			}
+
+			if obj.last != nil && result.Cmp(obj.last) == nil {
 				continue // value didn't change, skip it
 			}
-			obj.last = value // store so we can send after this select
+			obj.last = result // store so we can send after this select
 
 		case <-ctx.Done():
 			return nil
@@ -139,3 +144,13 @@ func (obj *ChannelBasedSinkFunc) Stream(ctx context.Context) error {
 		}
 	}
 }
+
+// Call this function with the input args and return the value if it is possible
+// to do so at this time.
+// XXX: Is is correct to implement this here for this particular function?
+func (obj *ChannelBasedSinkFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	if len(args) != 1 {
+		return nil, fmt.Errorf("programming error, can't find edge")
+	}
+	return args[0], nil
+}