Add Category instance for Flip

[masaeedu]

Description of the change

If p :: K -> K -> Type represents the hom-constructor of some category, then Flip p :: K -> K -> Type represents the hom-constructor of its dual category. This category has the same objects as p, but the set of arrows from some object A to some object B is taken to be the set of arrows from the object B to the object A in the original category.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation
• [ ] Added a test for the contribution (if applicable)