An initial implementation of unstructuring and structuring enums with complex values

[reversefold]

Fixes #699. The values of an Enum are unstructured and structured. Structuring is only done when the Enum class has a type defined for the _value_ attribute as per the Python typing standard.

I've added two very simple tests which show that this works. I'm attempting to add tests which use hypothesis like the rest but the magic chains of incantations in tests/typed.py are throwing me a bit and it's taking a while to understand what they're doing and how to use it to generate random dynamic Enum classes. Any pointers there would be appreciated.