perf: avoid clone on serialize frequency items

[tisonkun]

cc @k3dom

[k3dom]

Nice, LGTM!

Two small thoughts:

• active_entries() could probably be a lazy iterator impl Iterator<Item = (&T, u64)> + '_ instead of collecting Vec<(&T, u64)> which would make it totally allocation-free, not sure if it would be always faster though for very large maps.
• Would probably also make sense to update frequent_items_with_threshold as a follow-up because here we still clone every qualifying item into owned Row's before the caller can filter