nicer JSON for standard wrappers

Wrappers.proto defines types for nullable scalars. When those are mapped to JSON you get a nullable object with a value field. That's a bit awkward and unnecessary.
It would be nicer if the wrapper would be mapped to a simple nullable JSON field.
This could be special cased or maybe there is some way of marking wrappers. Initially just the well-known ones would be OK, I guess.

I could make a PR if you like.