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.
On the other hand, it's harder to see what's fundamental. It's easy to rewrite if we need. @kortschak , do you have an opinion?