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.

The issue names no files, tests, or entry points. Start by locating the existing Python pickling and Julia serialization implementations, then trace how objects cross the language boundary. Done means recursive Julia and Python objects can be serialized and deserialized through the proposed custom Pickler, Unpickler, and Serializer types.

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