Misc Missing Functionality for Roaring64 bitmaps

[Zachary Dremann]

This is just a list of functionality which I noticed was missing in comparison to roaring_bitmap_t as I was adding bindings to croaring-rs. I may add more if I find anything else, rather than making more issues.

• roaring64_bitmap_clear, to remove all data, and start from an empty bitmap
  (can this be done more efficiently than roaring64_bitmap_remove_range_closed(0, UINT64_MAX)). It seems like roaring64_bitmap_remove_range_closed could be optimized, it appears to loop through all high 48 bit combos, rather than skipping around using the ART. roaring64_bitmap_remove_range_closed(0, UINT64_MAX) is now efficient.
• The ability to do external iteration (an iterator type that the caller controls arbitrarily)
• roaring64_bitmap_remove_run_compression. This is probably only really useful after #542, to avoid run containers in the serialization when the reader won't understand them
• roaring64_bitmap_internal_validate, mostly useful for tests, to ensure our assumptions are never broken (max art depth, things which should be sorted are, etc).
• roaring64_bitmap_flip
• roaring64_bitmap_range_cardinality is exclusive only, it's unfortunate that roaring64_bitmap_cardinality(r) can't be emulated by roaring64_bitmap_range_cardinality(r, 0, UINT64_MAX)

[Tom Cornebize]

If I may add a few to the list:

• A function to convert a roaring64_bitmap_t to an array of uint64_t (similar to roaring_bitmap_to_uint32_array)
• A function to convert a roaring_bitmap_t to a roaring64_bitmap_t