Use cases: Implement marshal()/unmarshal() based on bundle::bundle() S3 methods

[HenrikBengtsson]

The bundle package has bundle() and unbundle() for marshalling and unmarshalling of model-based objects, cf. https://github.com/rstudio/bundle/tree/main/R. Implementing marshal() and unmarshal() S3 methods for those will be a good case study and add more test cases.

/ht @dfalbel /cc @simonpcouch @juliasilge

[juliasilge]

Looking interesting 👀 -- let us know if you have further questions!