[MRG+1] 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 other query operations cannot be easily vectorised, such as gathering the set of candidates per query (which differ in cardinality). Unvectorised operations make sparse matrix calculations particularly inefficient (because extracting a single row is not especially cheap).
Right I forgot about that. I suppose that you have run the tests many times and they never fail. If that's right, let's just put an inline comment that says that explicit seeding is not supported in old versions of scipy but that this test outcome should not be sensitive to the random data seeding.