make build_tags_taxonomy() use string comparisons
make build_tags_taxonomy() use string comparisons

building categories, according to NYTProf this code is run approx 141 million times. for me, this change reduced the taxonomy build times to 1/5 of the original time.

openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server