Cleanup Load and save functions

We currently have a mess of load and save functions. I think they can all be unified into just one load and one save function whose behaviour is determined by what kind of object it is called on.

Alternatively, save could be moved to a method of the Serializable abstract base class. This would simplify type determination somewhat, but the real issue is would we prefer the interface to be

hp.save(obj(args))

or

obj.save(args)