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.
It splits apart the interface provided by stat (in the sense that it has functions that can be called with particular parameters) from the fact that some of the functions are just thin wrappers around other ones.  If at a later time we want to change some implementations, we don't have to rewrite the benchmarks.

If users don't want to run all of the tests, they can restrict the benchmarks run with the regexp it uses to match benchmark functions, `go test -bench MeanVariance` in this case.