Copying a Roaring64MapSetBitForwardIterator into an iterator from another map does not work

[tudor]

Roaring64Map m1;
auto it = m1.begin();
// ...
Roaring64Map m2;
it = m2.begin();

This leads to trouble, because Roaring64MapSetBitForwardIterator's copy assignment does not copy the map reference. So if you then use it.move(<value>), map_iter is going to point into the wrong map (m1).

[tudor]

Fix in #590