Support serialising to named mappings

[juntyr]

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This PR includes four changes:

• Implement PythonizeListType for PyTuple, a small convenience that cannot be achieved outside the crate
• Add support for prebuilding mappings during serialisation, which delays the collection of mapping key-value pairs until all are known
• Add support for named mappings during serialisation, which adds a tiny default-implemented helper trait method to pass forward the struct / tuple / enum name to the mapping type
• Allow deserializing an enum from any mapping instead of just dicts

I have found them to be very useful to serialise to and from class-like Python types (e.g. namedtuple which keep more type information around).