Support serialising/pickling recursive types

This can be done by having a custom `Pickler` and `Unpickler` classes which take a Julia `Serializer` on construction which is used to serialize any Julia objects it sees in `persistent_id`. These classes are used when serializing a Python object.

Similarly we can have a custom `Serializer` type which takes a Python `Pickler` and `Unpickler` on construction which is used to (de-)serialize any Python objects it sees. This class is used when pickling/unpickling a Julia object.