Function pancake_engine_common::merging::merge_entry_iters

pub fn merge_entry_iters<'a, EntIter, Ent, K>(
    entry_iters: impl Iterator<Item = EntIter>
) -> impl 'a + Iterator<Item = Ent>
where
    EntIter: 'a + Iterator<Item = Ent>,
    Ent: 'a + Mergeable<K>,
    K: 'a + Ord,

K-merges and then dedupes the arg iters.

@arg entry_iters: An iterator of iterators of entrysets, where each entryset contains borrowable Ks and is internally sorted by K, from newer entryset to older entryset.