Add an extra field to [Category]

This way, [FunctorFromTerminal] is convertible with its opposite.

I'm not sure why Category.Equality got 4 seconds slower.  It's probably
because [destruct]ing [PreCategory]s is now more costly. :-/

After    | File Name                             | Before
-----------------------------------------------------------
0m24.20s | Category/Equality                     | 0m19.72s
0m23.00s | Adjoint/UnitCounitCoercions           | 0m22.59s
0m21.36s | Adjoint/Hom                           | 0m21.58s
0m12.20s | Adjoint/Composition                   | 0m12.79s
0m11.11s | Comma/Duals                           | 0m10.64s
0m07.98s | Comma/InducedFunctors                 | 0m08.37s
0m07.38s | Groupoid                              | 0m07.27s
0m06.44s | Adjoint/Equality                      | 0m06.71s
0m05.97s | Adjoint/Duals                         | 0m05.91s
0m05.47s | Category/Morphisms                    | 0m05.61s
0m04.91s | SumPreCategory                        | 0m04.78s
0m04.84s | Groupoid/Duals                        | 0m04.74s
0m04.36s | Groupoid/Functors                     | 0m04.06s
0m04.06s | Functor/Equality                      | 0m04.02s
0m03.36s | UniversalProperties                   | 0m03.24s
0m02.77s | Grothendieck/ToSet                    | 0m02.64s
0m02.53s | BoolCategory                          | 0m02.56s
0m02.47s | Comma/CommaCategory                   | 0m02.42s
0m01.87s | Category/Product                      | 0m01.92s
0m01.83s | NaturalTransformation/Duals           | 0m01.78s
0m01.80s | NaturalTransformation/Equality        | 0m01.78s
0m01.65s | Hom                                   | 0m01.69s
0m01.47s | NaturalTransformation/Product         | 0m01.48s
0m01.29s | Functor/Product                       | 0m01.35s
0m01.29s | NaturalTransformation/Composition     | 0m01.21s
0m01.22s | NaturalTransformation/CompositionLaws | 0m01.20s
0m01.20s | NaturalTransformation/Isomorphisms    | 0m01.24s
0m00.80s | Cat/Morphisms                         | 0m00.72s
0m00.72s | FunctorCategory/Morphisms             | 0m00.66s
0m00.65s | InitialTerminalCategory               | 0m00.72s
0m00.42s | Functor/CompositionLaws               | 0m00.45s
0m00.40s | Comma/Projection                      | 0m00.38s
0m00.37s | IndiscreteCategory                    | 0m00.30s
0m00.37s | Discrete/Duals                        | 0m00.40s
0m00.34s | SetCategory                           | 0m00.28s
0m00.34s | Cat                                   | 0m00.32s
0m00.32s | Common                                | 0m00.30s
0m00.30s | FunctorCategory                       | 0m00.30s
0m00.28s | Functor/Duals                         | 0m00.23s
0m00.28s | NaturalTransformation/Identity        | 0m00.34s
0m00.26s | Category/Objects                      | 0m00.26s
0m00.24s | Functor/Composition                   | 0m00.24s
0m00.22s | Adjoint/UnitCounit                    | 0m00.27s
0m00.20s | NatCategory                           | 0m00.24s
0m00.19s | IsGroupoid                            | 0m00.19s
0m00.18s | Category/Core                         | 0m00.18s
0m00.18s | NatCategory/Duals                     | 0m00.17s
0m00.18s | NaturalNumbersObject                  | 0m00.16s
0m00.16s | Adjoint                               | 0m00.14s
0m00.16s | Adjoint/Identity                      | 0m00.15s
0m00.16s | Category/Duals                        | 0m00.17s
0m00.16s | NaturalTransformation/Core            | 0m00.18s
0m00.14s | NaturalTransformation                 | 0m00.12s
0m00.14s | ComputableCat                         | 0m00.15s
0m00.13s | DiscreteCategory                      | 0m00.11s
0m00.13s | Functor/Core                          | 0m00.13s
0m00.11s | Functor/Identity                      | 0m00.08s
0m00.11s | Category/StrictCategory               | 0m00.08s
0m00.11s | Utf8                                  | 0m00.09s
0m00.11s | Peano                                 | 0m00.14s
0m00.10s | Functor                               | 0m00.09s
0m00.10s | Category/UnivalentCategory            | 0m00.09s
0m00.10s | Grothendieck                          | 0m00.10s
0m00.06s | Category                              | 0m00.08s
0m00.06s | NotationsUtf8                         | 0m00.05s
0m00.06s | Notations                             | 0m00.04s
-----------------------------------------------------------
2m57.37s | Total                                 | 2m52.39s