Add #[derive(View)] for enums and nested structs

[matthunz]

I think a derive macro for the view trait, especially on enums, would really benefit ergonomics.
Views that return multiple types on a condition, such as an if block or match statement, could instead return an enum with View derived.

For example, for conditional views we could return a custom enum like:

#[derive(View)]
enum UserForm {
  Login(LoginView),
  SignUp(SignUpView)
}

[xarvic]

Sorry for the slow response.
I like the idea to derive View for enums but am not so sure about structs. We already have implemented ViewSequence for Tuples. I don't see the benefit of implementing this for other product types.
The other thing to consider is that we already have AnyView which can do the same thing. And since this would be more of a performance improvement for platforms like embedded, it not a priority right now.