OpenAPI.Document is not a discriminated union and I wish it was one

### What happens? Per #157 this is kind of a feature request, but discussions aren't turned on. When I run a document through `deference` or `upgrade`, I get the `OpenAPI.Document` type. This type contains a bunch of union fields, like `.paths` is a union between all possible paths types:  It'd be ideal to: - Have this type be a discriminated union based on the value of `.openapi` - Have `upgrade` return a v3.1 type, because it should always be upgrading to that As it is, it's tricky to work with the types because they are too flexible. ### What did you expect to happen? These types would be discriminated ### How can we reproduce the issue? … ### Swagger/OpenAPI Example http://