bitmap: use external iterator

Reuse the Iterator object from hashmap.h and expose a similar API.

This allows us to do

{
    Iterator i;
    unsigned n;

    BITMAP_FOREACH(n, b, i) {
        Iterator j;
        unsigned m;

        BITMAP_FOREACH(m, b, j) {
            ...
        }
    }
}

without getting confused. Requested by David.