add a Functor structure

add a Functor structure

The intent is to replace some uses of Vec with a structure that also
happens to be a functor. RawFunctors from the standard library provide
no laws though, so we define a Functor structure that adds the laws. As
an additional law congruence is added, because

 * Other standard library structures in Algebra.Structures also require
   congruence.
 * Otherwise the identity law would have to reason about different
   identities.