CSV generation for large tuples (aka custom CSVItem conversion)

[kondziu]

When an entity is mapped into more than 11 elements, the csv generator functions don't understand the resulting tuple type. There should be a way to generate a CSV row (or multiple rows) from an entity in those cases.

One candidate solution is to have something like:

db.projects()
    .into_csv(|project| { 
        vec![CSVItem::from("id", project.id()),
               CSVItem::from("url", project.url())],
    }, ...)