Add Cat.

Actually, it's called SubPreCat for now, because it's a sub-precategory
of the precategory of precategories satisfying a given (h)Prop.  There's
also StrictCat, which still needs to be proven to be a strict category,
if it is, and I still need to define the category of categories, Cat.
But showing that [IsCategory] is an hProp is hard, so I've put it off
for now.

After    | File Name                             | Before
-----------------------------------------------------------
0m39.45s | Category/Equality                     | 0m40.71s
0m21.10s | NaturalTransformation/Equality        | 0m21.04s
0m17.36s | Functor/Equality                      | 0m17.46s
0m10.85s | Comma/Duals                           | 0m10.19s
0m10.83s | UniversalProperties                   | 0m10.65s
0m10.80s | Comma/CommaCategory                   | 0m10.90s
0m09.10s | Groupoid                              | 0m08.97s
0m07.90s | Category/Morphisms                    | 0m07.40s
0m04.68s | Grothendieck/ToSet                    | 0m04.58s
0m04.09s | Groupoid/Functors                     | 0m03.78s
0m02.27s | SumPreCategory                        | 0m02.28s
0m02.18s | Functor/CompositionLaws               | 0m02.12s
0m02.09s | Category/Product                      | 0m02.12s
0m01.73s | NaturalTransformation/Duals           | 0m01.84s
0m01.34s | NaturalTransformation/Composition     | 0m01.35s
0m01.17s | NaturalTransformation/CompositionLaws | 0m01.28s
0m01.08s | InitialTerminalCategory               | 0m01.14s
0m01.02s | Comma/Projection                      | 0m01.03s
0m00.98s | NatCategory                           | 0m00.90s
0m00.69s | BoolCategory                          | 0m00.61s
0m00.52s | SetCategory                           | 0m00.51s
0m00.50s | Cat                                   |   N/A
0m00.40s | Functor/Product                       | 0m00.37s
0m00.39s | Discrete/Duals                        | 0m00.42s
0m00.32s | Common                                | 0m00.33s
0m00.31s | IndiscreteCategory                    | 0m00.30s
0m00.26s | NaturalTransformation/Identity        | 0m00.25s
0m00.24s | Functor/Duals                         | 0m00.26s
0m00.24s | Adjoint/UnitCounit                    | 0m00.28s
0m00.22s | IsGroupoid                            | 0m00.21s
0m00.22s | FunctorPreCategory                    | 0m00.23s
0m00.22s | NatCategory/Duals                     | 0m00.22s
0m00.21s | Functor/Composition                   | 0m00.25s
0m00.20s | Category/Core                         | 0m00.13s
0m00.20s | Category/Objects                      | 0m00.25s
0m00.16s | NaturalTransformation/Core            | 0m00.17s
0m00.14s | NaturalTransformation/Product         | 0m00.14s
0m00.13s | ComputableCat                         | 0m00.10s
0m00.11s | Category/Duals                        | 0m00.14s
0m00.11s | Peano                                 | 0m00.12s
0m00.10s | NaturalTransformation                 | 0m00.11s
0m00.10s | Functor/Core                          | 0m00.17s
0m00.09s | Category/Category                     | 0m00.09s
0m00.09s | Functor                               | 0m00.08s
0m00.08s | Category/StrictCategory               | 0m00.08s
0m00.08s | Utf8                                  | 0m00.09s
0m00.08s | Grothendieck                          | 0m00.08s
0m00.07s | Functor/Identity                      | 0m00.08s
0m00.07s | DiscreteCategory                      | 0m00.10s
0m00.04s | NotationsUtf8                         | 0m00.05s
0m00.04s | Notations                             | 0m00.05s
-----------------------------------------------------------
2m36.65s | Total                                 | 2m36.01s