Add a proof that groupoids are self-dual

Thanks to Dan Licata for some help.

After    | File Name                             | Before
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0m39.50s | Category/Equality                     | 0m40.51s
0m22.09s | NaturalTransformation/Equality        | 0m22.03s
0m17.10s | Functor/Equality                      | 0m17.28s
0m11.38s | Comma/CommaCategory                   | 0m11.52s
0m11.03s | UniversalProperties                   | 0m10.92s
0m10.49s | Comma/Duals                           | 0m10.56s
0m09.12s | Groupoid                              | 0m08.86s
0m07.87s | Category/Morphisms                    | 0m07.78s
0m04.80s | Grothendieck/ToSet                    | 0m04.66s
0m03.82s | Groupoid/Functors                     | 0m03.88s
0m02.35s | Groupoid/Duals                        |   N/A
0m02.18s | Functor/CompositionLaws               | 0m02.23s
0m02.17s | SumPreCategory                        | 0m02.43s
0m02.08s | Category/Product                      | 0m02.13s
0m01.79s | NaturalTransformation/Duals           | 0m01.74s
0m01.34s | NaturalTransformation/Composition     | 0m01.40s
0m01.18s | NaturalTransformation/CompositionLaws | 0m01.22s
0m01.10s | InitialTerminalCategory               | 0m01.13s
0m01.01s | NatCategory                           | 0m00.85s
0m00.94s | Comma/Projection                      | 0m01.06s
0m00.60s | BoolCategory                          | 0m00.66s
0m00.48s | SetCategory                           | 0m00.54s
0m00.44s | Common                                | 0m00.34s
0m00.42s | Cat                                   | 0m00.49s
0m00.40s | FunctorCategory                       | 0m00.45s
0m00.40s | Discrete/Duals                        | 0m00.42s
0m00.36s | Functor/Product                       | 0m00.36s
0m00.31s | IndiscreteCategory                    | 0m00.34s
0m00.27s | Adjoint/UnitCounit                    | 0m00.26s
0m00.26s | Category/Objects                      | 0m00.21s
0m00.26s | NaturalTransformation/Identity        | 0m00.28s
0m00.24s | Functor/Composition                   | 0m00.21s
0m00.24s | NatCategory/Duals                     | 0m00.22s
0m00.22s | Functor/Duals                         | 0m00.24s
0m00.21s | IsGroupoid                            | 0m00.25s
0m00.17s | Category/Duals                        | 0m00.18s
0m00.16s | Category/Core                         | 0m00.18s
0m00.16s | NaturalTransformation/Core            | 0m00.18s
0m00.13s | ComputableCat                         | 0m00.11s
0m00.13s | Functor/Core                          | 0m00.13s
0m00.13s | NaturalTransformation/Product         | 0m00.16s
0m00.12s | Peano                                 | 0m00.10s
0m00.11s | Category/Category                     | 0m00.10s
0m00.11s | DiscreteCategory                      | 0m00.08s
0m00.10s | Functor/Identity                      | 0m00.14s
0m00.10s | Functor                               | 0m00.08s
0m00.09s | NaturalTransformation                 | 0m00.10s
0m00.09s | Category/StrictCategory               | 0m00.07s
0m00.08s | Utf8                                  | 0m00.09s
0m00.08s | Grothendieck                          | 0m00.10s
0m00.05s | NotationsUtf8                         | 0m00.03s
0m00.05s | Notations                             | 0m00.04s
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2m40.30s | Total                                 | 2m39.32s