implement a custom save function for R6 objects to avoid slow serialisation.

save R6 objects as RDS can take very long time to load.

Ideas:
Save all data objects individually. Ideally, we want to avoid *.rds for large objects. Maybe giving the user choices of backends (fst, feather, qs) would be good.  

Things to beware of:
- A world object has fields that point to other objects in the memory hence this will be quite hard to rebuild all linked objects correctly.