Implementing visibility / additional metadata for objects

In order to mark an object as visible / invisible (iirc, print currently prints twice), some additional meta-data is needed to store such information. I have seen that this was started to be implemented as part of the `Return` variant of the `Signal` enum (https://github.com/dgkf/R/blob/006f1a4b2fefa9bf1043076d449ee9026e4e10c9/src/lang.rs#L39). However, maybe it makes sense to take this as a starting point to introduce the possibility for some metadata for the `Obj` class? Because there will be other use-cases to store bits alongside R objects, an example being whether a vector is sorted.