r64 iterator: allow moving backward with move_equalorlarger

[SLieve]

Fixes #575. This adds the ability to move backward to both the ART iterator and the roaring64 iterator.

[Dr-Emann]

@SLieve Should roaring64_iterator_move_equalorlarger work if the iterator is exhausted (it doesn't currently)?

E.g. should this work (as long as the bitmap is not empty)?:

while (roaring64_iterator_advance(it))
    ;
assert_true(roaring64_iterator_move_equalorlarger(it, 0));

[SLieve]

Interesting question, I don't think see a reason why it shouldn't work so let's make it work.