EquivFunctor instances

We derive some EquivFunctor instances, to enable equiv_rw to rewrite under these functions.

instance EquivFunctorUnique : EquivFunctor Unique

instance EquivFunctorPerm : EquivFunctor Equiv.Perm

instance EquivFunctorFinset : EquivFunctor Finset

instance EquivFunctorFintype : EquivFunctor Fintype