[Merged by Bors] - feat(CategoryTheory): the category of commutative squares

[joelriou]

In this PR, we define a bundled version of CommSq which allows to consider commutative squares as objects in a category Square C.

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[github-actions]

PR summary 3e211eba44

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.Square	272

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Declarations diff

+ Hom
+ Square
+ arrowArrowEquivalence
+ arrowArrowEquivalence'
+ category
+ commSq
+ comp
+ flip
+ flipEquivalence
+ flipFunctor
+ fromArrowArrowFunctor
+ fromArrowArrowFunctor'
+ hom_ext
+ id
+ isoMk
+ toArrowArrowFunctor
+ toArrowArrowFunctor'

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

[erdOne]

I was just thinking about suggesting the same thing when I saw the other PR.
LGTM modulo minor comments.

[joelriou]

Thanks for the reviews @erdOne! Indeed, while finding the better way to formalize Mayer-Vietoris squares, it has become more obvious to me that introducing this category Square C was the better approach.

[erdOne]

Thanks!
maintainer merge

[github-actions]

🚀 Pull request has been placed on the maintainer queue by erdOne.

[riccardobrasca]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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