Support serialising/pickling recursive types

[cjdoris]

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.