feat(CategoryTheory): constructor for abelian categories

[joelriou]

We assume that the category C is preadditive, has finite products, and that any morphism f : X ⟶ Y has a kernel i : K ⟶ X, a cokernel p : Y ⟶ Q such that f factors as f = π ≫ ι where π : X ⟶ I is a cokernel of i and ι : I ⟶ Y is a kernel of p.

This will be used in order to show that the heart of a t-structure is an abelian category.

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[github-actions]

PR summary 99884c2730

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ mk'

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>

The doc-module for scripts/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).