validating structs directly doesn't work

Validation attempts to work on Go structs directly. But I think this is doomed:
- We would need to interpret some `json` struct tags (see #22)
- Worse, we need to somehow understand custom marshalers (types that implement `json.Marshaler`).

I believe the only general solution is to marshal the struct to JSON before validating.

I think we can still validate non-structs directly.