LSHForest: sparse support and vectorised _find_longest_prefix_match

This adds sparse matrix support to `LSHForest`, vectorises calls to `hasher.transform`, and vectorises `_find_longest_prefix_match` over queries. These seem to speed things up a little, but the benchmark script does not provide very stable timings for me.

Some operations cannot be easily vectorised, such as gathering the set of candidates per query (which differ in candidate set cardinality). This is followed by calculating exact distances from each query to its candidates, which may also not be trivial to do vectorised, and certainly not with `pairwise_distances`. Unvectorised operations make sparse matrix calculations particularly inefficient (because extracting a single row is not especially cheap).