Breaking change in W::bits method

[couchand]

I changed the W::bits method signature in #465 at d9e06a7#diff-b0b528563af56ba9f8af6cb6bb3d2539L213. It really looks like an oversight, but offhand I can't recall if there was a specific reason to do it. I didn't call it out in the PR, which makes me think it was a mistake.

In any case, I'd better fix it.