Generate a OneOf type based on an existing Enum

[wtaisto]

To promote interacting with existing Enum's using the OneOf paradigm, it might be nice to be able to apply a generic GenerateOneOf attribute to a class, which would wrap each of the Enum type's values in it's own struct, and then compose a OneOf implementation which will select / match against those wrapped struct types. This OneOf type could have a helper to instantiate it from a value from the underlying Enum.