Generics for bytezap's struct serializer.

We can't use my generic-data-functions library, because we're doing more than just basic monoidal composition. But I still want the same pluggable generics, where the user provides the class to use for base cases. So I do that. However, unlike g-d-f, the class info can't be provided via the user-selected monoid, because you don't select that. Instead, we take a simple "index" type. It's pretty much the same idea, surprisingly. This way, we can provide a few sensible "versions" like in g-f-d, while primarily designing for DIY.