Implement Hash for RoaringBitmap and RoaringTreemap - #365

Closes #231.

RoaringBitmap and RoaringTreemap implement Eq but not Hash, so they can't be used as keys in a HashMap or HashSet.

A derive won't work for RoaringBitmap, because two bitmaps holding the same values are equal even when their containers use different internal representations (array, bitmap or run). The hash is built from the values via the existing iterator so it stays consistent with PartialEq whatever the layout. RoaringTreemap just derives it, as suggested in the issue, since it's a BTreeMap of bitmaps.

Tests live in tests/hash.rs and tests/treemap_hash.rs and cover the case where the same values are stored in different representations (optimized vs not) and still hash equal.