refactor(MeasureTheory): golf Mathlib/MeasureTheory/Function/StronglyMeasurable/Basic

[yuanyi-350]

• refactors stronglyMeasurable_in_set to build the restricted approximation sequence inline
• replaces the auxiliary hg_eq and hg_zero lemmas with direct simpa/simp arguments on membership in s

Extracted from #38104

[yuanyi-350]

!bench

[leanprover-radar]

Benchmark results for 2c878be against d104ce7 are in. No significant results found. @yuanyi-350

• 🟥 build//instructions: +27.0G (+0.02%)

No significant changes detected.

[github-actions]

PR summary d104ce7317

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

No declarations were harmed in the making of this PR! 🐙

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.

---

No changes to technical debt.

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).

[yuanyi-350]

I ran a profiler comparison for the changed declarations in this PR.

Results (seconds):

• MeasureTheory.StronglyMeasurable.stronglyMeasurable_in_set: 0.025840 -> 0.325675 (1160.4%)

Overall:

• stronglyMeasurable_in_set regressed noticeably in elaboration time.