refactor: golf only Function/SimpleFunc, Function/LpSeminorm/Basic

[yuanyi-350]

This PR selectively ports part of #38104.

Concretely, it does two things:

• In Mathlib/MeasureTheory/Function/SimpleFunc.lean, it rewrites Measurable.add_simpleFunc and Measurable.simpleFunc_add to use SimpleFunc.measurable_bind instead of explicit inductions over simple functions together with manual piecewise/ support arguments.
• In Mathlib/MeasureTheory/Function/LpSeminorm/Basic.lean, it shortens several eLpNorm/eLpNorm' monotonicity and bound proofs by deriving the nnnorm versions from the more general enorm versions, instead of reproving the same argument separately.

[github-actions]

PR summary 8e3c989104

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

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

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

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

---

No changes to technical debt.

You can run this locally as

./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).

[grunweg]

A couple of minor comments. Can you comment on the performance aspects of this change?

[grunweg]

Echoing my comment made elsewhere, can you also update the PR description to be more helpful? Thanks!

[yuanyi-350]

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

Results (seconds):

• MeasureTheory.eLpNorm'_mono_nnnorm_ae: 0.159290 -> 0.056959 (-64.24%)
• MeasureTheory.eLpNorm_mono_nnnorm_ae: 0.063250 -> 0.010056 (-84.10%)
• MeasureTheory.eLpNorm_le_of_ae_nnnorm_bound: 0.218913 -> 0.082531 (-62.30%)
• Measurable.add_simpleFunc: 0.183754 -> 0.024048 (-86.91%)
• Measurable.simpleFunc_add: 0.165129 -> 0.019816 (-88.00%)

Overall:

• All 5 changed declarations got faster.
• No regressions showed up in the modified declarations I checked.

Caveat:

• I measured each declaration by inserting local set_option trace.profiler true and taking the top-level declaration/proof timing emitted for that declaration.
• The comparison uses the actual merge-base with upstream/master.

[grunweg]

Please follow the request made in the comments here and here. Thanks!