chore(CategoryTheory/Limits/Shapes/BinaryProducts): use to_dual

[JovanGerb]

This PR uses to_dual on binary products/coproducts.

• BinaryFan.fst/snd have been given an explicit type annotation so that the type doesn't involve Functor.const.

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[github-actions]

PR summary 3cbcf125ae

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 (regex)

+ BinaryFan.ext_inv_hom
+ Discrete.Hom.casesOn'
- BinaryCofan
- BinaryCofan.IsColimit.hom_ext
- BinaryCofan.IsColimit.mk
- BinaryCofan.ext
- BinaryCofan.ext_hom_hom
- BinaryCofan.inl
- BinaryCofan.inr
- BinaryCofan.isColimitMk
- BinaryCofan.mk
- BinaryCofan.mk_inl
- BinaryCofan.mk_inr
- BinaryCofan.ι_app_left
- BinaryCofan.ι_app_right
- isoBinaryCofanMk

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean -- pending)

Computed after the build finishes.

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Decrease in strong tech debt: (relative, absolute) = (1.00, 0.00)

Current number	Change	Type (strong)
backward.defeqAttrib.useBackward	4245	-1

No changes to weak technical debt.

Current commit 3cbcf125ae
Reference commit 059a625dc0

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.py 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).