Add definition of [unique] from std lib
Define duals for functor and nt

Also, add a symmetrized field to natural transformation, so that the
dual lemma goes through without functional extensionality.  It slows
down NaturalTransformation/Equality slightly, but, *shrug*.

After    | File Name                             | Before
-----------------------------------------------------------
0m39.95s | Category/Equality                     | 0m40.34s
0m21.29s | NaturalTransformation/Equality        | 0m15.66s
0m17.19s | Functor/Equality                      | 0m17.70s
0m10.93s | Comma/CommaCategory                   | 0m10.89s
0m09.21s | Groupoid                              | 0m09.61s
0m07.89s | Category/Morphisms                    | 0m07.61s
0m04.58s | Grothendieck/ToSet                    | 0m04.79s
0m04.00s | Groupoid/Functors                     | 0m03.82s
0m02.24s | SumPreCategory                        | 0m02.29s
0m02.09s | Functor/CompositionLaws               | 0m02.10s
0m01.84s | ProductPreCategory                    | 0m01.90s
0m01.68s | NaturalTransformation/Duals           |   N/A
0m01.33s | NaturalTransformation/Composition     | 0m01.35s
0m01.15s | NaturalTransformation/CompositionLaws | 0m01.10s
0m01.12s | InitialTerminalCategory               | 0m01.09s
0m00.83s | NatCategory                           | 0m00.84s
0m00.67s | BoolCategory                          | 0m00.65s
0m00.54s | SetCategory                           | 0m00.53s
0m00.31s | Common                                | 0m00.30s
0m00.28s | NaturalTransformation/Identity        | 0m00.24s
0m00.25s | FunctorPreCategory                    | 0m00.23s
0m00.22s | Functor/Duals                         |   N/A
0m00.21s | Functor/Composition                   | 0m00.26s
0m00.19s | Category/Core                         | 0m00.17s
0m00.19s | Category/Objects                      | 0m00.21s
0m00.16s | Category/Duals                        | 0m00.18s
0m00.15s | Peano                                 | 0m00.14s
0m00.14s | NaturalTransformation/Core            | 0m00.14s
0m00.13s | Functor/Core                          | 0m00.13s
0m00.12s | ComputableCat                         | 0m00.14s
0m00.10s | Category/Category                     | 0m00.11s
0m00.10s | Functor                               | 0m00.08s
0m00.09s | Grothendieck                          | 0m00.08s
0m00.08s | Functor/Identity                      | 0m00.09s
0m00.08s | Category/StrictCategory               | 0m00.09s
0m00.08s | NaturalTransformation                 | 0m00.09s
0m00.08s | DiscreteCategory                      | 0m00.09s
0m00.07s | Utf8                                  | 0m00.09s
0m00.05s | NotationsUtf8                         | 0m00.06s
0m00.05s | Notations                             | 0m00.04s
-----------------------------------------------------------
2m11.65s | Total                                 | 2m05.23s