Send/Sync implementation for Bitmap

[hashmap]

Is Bitmap Send/Sync-able? It seems to be according to https://github.com/RoaringBitmap/CRoaring#thread-safety

Thus whenever you modify a bitmap in one thread, it is unsafe to query it in others. It is safe however to query bitmaps (without modifying them) from several distinct threads, as long as you do not use the copy-on-write attribute.

I'm not familiar with the internals, so may be missing something