chore(CategoryTheory/Functor): more API for dense functors

[chrisflav]

Given a dense functor F : C ⥤ D, we provide an isomorphism F.leftKanExtension F ≅ 𝟭 D. We also provide the instance that yoneda is dense.

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[github-actions]

PR summary 8f3bdb18b4

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

+ DenseAt.hasPointwiseLeftKanExtensionAt
+ IsDense.leftKanExtensionIso
+ IsDense.leftKanExtensionUnit_leftKanExtensionIso_hom
+ IsDense.leftKanExtensionUnit_leftKanExtensionIso_hom_app
+ denseAtUliftYoneda
+ denseAtYoneda
+ instance (F : C ⥤ D) [F.IsDense] : F.HasPointwiseLeftKanExtension F
+ instance (F : C ⥤ D) [F.IsDense] : Functor.IsLeftKanExtension (𝟭 D) (Functor.rightUnitor F).inv
+ instance : (uliftYoneda.{w} (C := C)).IsDense
+ instance : (yoneda (C := C)).IsDense

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.

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No changes to strong technical debt.

No changes to weak technical debt.