roaring: improve SliceAscending/SliceDescending tests
roaring: improve SliceAscending/SliceDescending tests

Two changes: First, make SliceDescending set the entire
slice, not all-but-one bits. Second, add tests that are
"striped", so it's writing to 8 parts of the slice
sequentially, rather than just going up or down the whole
thing, because that gives us some cheap indication of
cache-locality impact, which turns out to be possibly
significant.

travisturner/pilosa