Simplify sigs

As discussed at https://groups.google.com/forum/#!topic/gonum-dev/nOWoTO2ilVM

This is more of a request for comment than something I want to merge right away.  In particular, I've changed some of the algorithms to use a correction.
The functions benchmarked are all of the moment measurements, and are a sensible logical grouping, so I named the file after them.  If you'd like me to rename it, that's fine.  The file will get a bit long if all benchmarks for the stats.go file end up in it though.