Misc Missing Functionality for Roaring64 bitmaps

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_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)
• roaring64_bitmap_steal_roaring32(roaring_bitmap_t *) which steals the containers from the passed 32 bit bitmap. (naming?) #649
• roaring64_bitmap_remove_run_compression
• roaring64_bitmap_add_offset
• roaring64_bitmap_overwrite
• CoW support (not sure this is something we even want to support?)
• Debug Printing (roaring64_bitmap_printf/roaring64_bitmap_printf_describe)
• Iterator skipping (roaring64_iterator_skip/roaring64_iterator_skip_backward)

Start by reviewing the three unchecked entry points: CoW support, debug printing through roaring64_bitmap_printf or roaring64_bitmap_printf_describe, and iterator skipping through roaring64_iterator_skip or roaring64_iterator_skip_backward. Compare their intended behavior with the corresponding roaring_bitmap_t functionality and the croaring-rs bindings, then define a focused scope before implementing anything.