bitmap: fix bitmap_equal on bitmaps with unset bits

bitmap: fix bitmap_equal on bitmaps with unset bits

Given two bitmaps and the following code:

Bitmap *a = bitmap_new(), *b = bitmap_new();
bitmap_set(a, 0);
bitmap_unset(a, 0);

These two bitmaps should now have the same bits set and they should be
equal but bitmap_equal() will return false in this case because the
bitmaps array in a is larger because of the bit which was previously
set.

Fix this by comparing only the bits which exists in both bitmaps and
then check that the rest of the bits (if any) is all zero.

This also adds test code for this issue.