Roaring structs as value types

Currently, all functions take Roaring objects as pointer types... and all internal containers are effectively pointers on the heap.

There is an unmerged (API breaking) pull request that allows you to use Roaring structs as value types instead of pointer types...

https://github.com/RoaringBitmap/roaring/pull/149

We have opted not to merge this PR since it breaks the API and that's just bad for our users.

It looks like it massively speeds things up... (see the numbers at https://github.com/RoaringBitmap/roaring/pull/149) so we do not want to drop this idea.

My current proposal to get around this is to introduce a new struct and then basically do some copy and paste to recover @anacrolix's code. This would add to our API instead of changing it, so it would be safe. Since it introduces very few changes, it would not increase our maintenance burden significantly.

No source files or tests are named; start by reviewing the current public API and the unmerged pull request #149. Define the proposed additional value-type struct without changing existing APIs, then verify that compatibility is preserved and the reported performance improvement is supported by tests or benchmarks.