Support conditional flattening

[ghost]

#[derive(Deserialize)]
struct Response<T> {
    #[serde(flatten_if = "Result::is_err")]
    result: Result<T, ResponseErr>,
}

[dtolnay]

I think I would prefer not to build this directly into serde_derive, but it would be reasonable for https://github.com/jonasbb/serde_with or a similar library to provide this functionality.