Function group_entries_by 

pub fn group_entries_by<'a, K, F>(
    entries: &'a [Entry],
    key_fn: F,
) -> Vec<(K, Vec<&'a Entry>)>
where
    K: Eq + Hash,
    F: Fn(&'a Entry) -> K,

Group entries by an arbitrary key, preserving the order keys are first seen.