Merge: src/model/model_index: model index uses BKTree

This PR makes ModelIndex use a BKTree (#2718) to speed up Nitweb name similarity search queries on large code corpora.

Levenstein names comparison with 300k+ entities took a bit long (up to 10s for Nitweb with `contrib` and `src` for some queries). The use of BKTree speeds things up and brings those queries under 0.1s.

Pull-Request: #2724
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/core/collection/union_find.nit
Simple merge