ListKind: Use an exhaustive match to future-proof.

    /// Return true if this source indicates a guard that came from a sample

    /// taken from a network directory at some point.

    pub(crate) fn is_guard_sample(&self) -> bool {

        self != &ListKind::Fallback

        match self {

            ListKind::Primary | ListKind::Confirmed | ListKind::Sample => true,

            ListKind::Fallback => false,

        }

    }

}