Allow Vec<(Int, Int)> for struct fields

[kreibaum]

Right now, this type can not be exported:

pub struct ProjectorView {
    pub question: Option<QuestionView>,
    pub connected_users: Vec<String>,
    pub high_scores: Vec<(String, usize)>,
}

thread 'main' panicked at 'not yet implemented: Tuple missing', src/lib.rs:590:32
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This seems to be a type I actually like to use sometimes so I should add this.

Workaround is a custom struct type / type alias.