Add a roaring64 iterator and functions for it

[SLieve]

For issue #549: "The ability to do external iteration (an iterator type that the caller controls arbitrarily)".

I decided not to expose the iterator type, because it would need to pull in a lot of stuff from ART. I'm open to changing this though.

[Dr-Emann]

@SLieve

This seems to imply (to me) that something like this should work

roaring64_bitmap_t *b = roaring64_bitmap_of(1, 100ULL);
roaring64_iterator_t *it = roaring64_iterator_create(b);
// it points at the first item, 100

roaring64_iterator_previous(it);
// it now points before the first item

roaring64_iterator_advance(it);
assert(roaring64_iterator_has_value(it));
assert(roaring64_iterator_value(it) == 100);

But instead it fails, right at the beginning of roaring64_iterator_advance:

if (it->art_it.value == NULL) {
    return (it->has_value = false);
}

In fact, because of this check, it seems like it should be safe to call roaring64_iterator_advance repeatedly even after reaching the end of the bitmap.

[SLieve]

Yes I'd consider not being able to go back when reaching the end incorrect behavior, we should fix that.

[Dr-Emann]

@SLieve roaring_uint32_iterator_move_equalorlarger is able to move the iterator forward/backward, is the difference between that and roaring64_iterator_move_equalorlarger (which can only move forward) on purpose?

[SLieve]

I hadn't realized that the 32-bit version was able to go in both directions. I did this to simplify the implementation, but for consistency we should make it go in both directions.