Add variant types

[DKLoehr]

It would be useful to be able to declare OCaml-style variant types in NV. Of course, they could not be recursive.

In fact, all the use cases I've seen have required only enum types (i.e. variant types with only constant constructors, no constructors which carry values). This would be a good place to start, since it's simpler and seems likely to get us most (or all) of what we want.