Type-level equivalent of a subset of Num.

This provides "kindclasses" (actually open type families) with functionality analogous to that provided by the Num typeclass. Since type-level typeclasses don't exist, instead we translate each would-be method to its own open type family; then "instances" are implemented by providing clauses for each type family "method". Unfortunately this means we can't group methods into classes that must be implemented all-or-none, but in practice this seems to be okay.