[Merged by Bors] - chore: golf using simp

[euprunin]

The goal of this PR is to decrease the number of times lemmas are called explicitly. Any decrease in compilation time is a welcome side effect, although it is not a primary objective.

Trace profiling results (differences <30 ms considered measurement noise):

• ✅️ MvPolynomial.coeff_sum_X_pow_of_fintype: 430 ms before, 101 ms after 🎉
• ✅️ CategoryTheory.Functor.Monoidal.associator_hom_app: 59 ms before, <30 ms after 🎉
• ✅️ Height.mulHeight₁_div_eq_mulHeight: unchanged 🎉
• ✅️ Ordinal.nfp_add_zero: unchanged 🎉

Profiled using set_option trace.profiler true in.

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[github-actions]

PR summary b6118399e6

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

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.

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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]

Thanks!
bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

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