This test was supposed to check against all the container types,
but especially bitmaps, but turns out not to work because the
containers turn into RLE containers. Oops. Now, we start with
every-other-bit for the first 8k, then start filling in the holes,
so we get some bitmap containers and then start generating
RLE containers.
On roaring, CountRange needs to have exclusive access to a fragment, but
doesn't currently require a lock, because it's usually used from inside
other already-locked things.
CountRange for RBF had a subtle bug which wasn't noticed, so, let's
have some CountRange testing and also a benchmark.
We also fix a couple of subtle bugs caught in the process of developing
and testing this.
SliceContainers will allow nil containers, but doesn't return them when
iterating because there's various things that can panic if called on a nil
container. Since countEmptyContainers() has to traverse the whole bitmap
anyway, it doesn't matter which it counts, so we replace it with
countNonEmptyContainers(), and adjust test cases accordingly. This fixes
an issue where if roaring is smart enough to insert a nil container
into a SliceContainers, trying to write it to a file produces an invalid
bitmap with offsets off by 16 and one container fewer than its header predicts.
RBF: don't try to count 0 bits in a container
If we're to the "last container", and we'd be counting all the bits less than
zero, we can skip that. This avoids hitting a bug, which is that c.countRange
doesn't handle BitmapPtr.
