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_source.go

@@ -113,3 +113,13 @@ func (obj *ChannelBasedSourceFunc) Stream(ctx context.Context) error {
 		}
 	}
 }
+
+// XXX: Is is correct to implement this here for this particular function?
+// XXX: tricky since this really receives input from a secret channel...
+// XXX: ADD A MUTEX AROUND READING obj.last ???
+//func (obj *ChannelBasedSourceFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+//	if obj.last == nil {
+//		return nil, fmt.Errorf("programming error")
+//	}
+//	return obj.last, nil
+//}