keywords in nominal.clj.
Remove `FDConstraint` wrapper type. This mostly meant that the fd constraints needed to implement `IEnforceableConstraint` and `IConstraintWatchedStores`, most already implemented `IRunnable` of course. Add `id` and `with-id` fns which delegate to `-id` and `-with-id` if the constraint implements them. This is prep to see if we can remove those protocol fns without taking a performance hit. This would simplify constraint definition and leave the door open for variadic constraints - currently the final argument must be taken by the constraint id.