Method progress

src/surface/tutorial.rs:20–27

(&self)

impl Tutorial {
    pub fn progress(&self) -> usize {
        for (idx, item) in Tutorial::iter().enumerate() {
            if *self == item {
                return idx;
            }
        }
        unreachable!()
    }

    pub fn next(&mut self) {
        let n = self.progress() + 1;