Fix review comments from <https://info.ravenbrook.com/mail/2013/05/28/22-21-11/0/>:
* Rename RangeOverlap to RangesOverlap.
* MVFF shouldn't assume that CBSInsert and CBSDelete can't fail.
* Remove unused function mps_mvff_stat.
* Don't call range methods in RangeCheck.
* RangeInit can't fail, so return void.