Function optional

pub fn optional<'outer, S, R>(
    f: impl for<'inner> FnOnce(&mut Optional<'outer, 'inner, S>) -> R,
) -> R

This is a combinator function that allows constructing single row optional queries.

let res = txn.query_one(optional(|row| {
    let x = row.and(Some("test"));
    let y = row.and(Some(42));
    row.then((x, y))
}));
assert_eq!(res, Some(("test".to_owned(), 42)));

let res = txn.query_one(optional(|row| {
    let x = row.and(Some("test"));
    let y = row.and(None::<i64>);
    row.then((x, y))
}));
assert_eq!(res, None);