chore: move strong measurability proof to setToFun

[sgouezel]

The goal is to be able to use the very same lemma for the vector measure integral, in a forthcoming PR.

---

[github-actions]

PR summary a6b367d2de

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ StronglyMeasurable.setToFun_prod_right
+ _root_.measurableSet_integrable
+ setToFun_simpleFunc_eq_setToSimpleFunc
+ setToSimpleFunc_eq_sum_of_subset
- measurableSet_integrable

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)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit a6b367d).

• +3 new declarations
• −0 removed declarations

+MeasureTheory.SimpleFunc.setToSimpleFunc_eq_sum_of_subset
+MeasureTheory.StronglyMeasurable.setToFun_prod_right
+MeasureTheory.setToFun_simpleFunc_eq_setToSimpleFunc

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit a6b367d2de
Reference commit 6923f2f175

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.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).