Redo axiom rules, so that they just keep a FastString rather than a Name.

This is simpler, and sufficient, because CoAxiomRules are only ever
built into the compier, and there is no way for the user to introduce
them (at least, not yet!)
Fix typo in declaration of exponentiation constructor.
Just a comment to make it clear that AxiomRuleCo should be fully saturated.
Generalize CoAxiomRule to support non-nominal Roles.  Add coreLint checks.