JLD2 compatibility

Since JLD2 serialises all pointers to NULL, serializing a `Py` always returns `PyNULL`.

Add a package extension with a custom serialiser using `pickle`.

Possibly have the deserialiser be opt-in, for safety by default? Same for Serialization?

Start by reviewing the JLD2 compatibility request and the package's extension mechanisms. Define the custom pickle-based serializer for Py values, then resolve whether deserialization and Serialization support must be opt-in for safety; done means Py values no longer become PyNULL when serialized and the chosen safety behavior is covered.

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