num/{dual,hyperdual}: new packages for dual and hyperdual arithmetic

Created by: kortschak

As suggested in https://github.com/gonum/gonum/pull/587#issuecomment-420165610.

This does not add parsing as it seems likely that it would be less useful here. I'm happy to revisit the naming of fields (and packages/types), and there are special cases that are not properly handled (some listed, but probably others as well).

Please take a look.