Cleanup roaring integration code

Mainly fix code style, rename variables, clean up some logic
Apologize for the rapid-fire succession of conflicting comments, the branch was still on 0.4.1, casting works fine with both versions. My only reason for having a guaranteed immutable copy would be in the event that multiple threads operate simultaneously on the same bitmap and we require a copy at a given point in time that would not be affected by any parallel operation on the mutable reference kept by other threads.