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.

[Tinche]

Thanks for working on this, I will review within a couple days when I get some time. Don't worry about Hypothesis, I'm kind of moving away from it, since it bloats the test times and can turn out to be very complex to use properly, so the cost/benefit ratio is usually in favor of just normal unit tests. :)