The main idea is to no longer need to implement Serializable for composite keys, i.e. avoid to complain at boot time with messages like:

> org.hibernate.MappingException: Composite-id class must implement Serializable:...

We might also want to relax some APIs which accept only Serializable to for an ID, such as load(Class, Serializable id), however that's less valuable as the user's code wouldn't compile (so there are no surprises at boot time).