Detect special type cases in python deserialisation

[aborgna-q]

Convert Sums into UnitSum / Tuple / Option / Result as appropriate.

Auto detection on round trip was left as a todo/future consideration because it also has implications how how we treat implementer of AsExtOp. Said operations would also have to register themselves with deserialization. Probably worth making an issue about this to discuss.

Originally posted by @ss2165 in #1481 (comment)

[ss2165]

A related problem is when different instances are equivalent in internal representation. I.e. if a user specified a Result<T, ()> should a roundtrip happily turn that in to Option<T>? Not ideal.

[aborgna-q]

Not ideal, but it's unavoidable unless we want to encode every different specialisation as serialisation variants.