[Merged by Bors] - chore: golf proofs

[euprunin]

---

Trace profiling results (shown if ≥10 ms before or after):

• SimpleGraph.Walk.nil_takeUntil: 88 ms before, <10 ms after 🎉
• Monoid.CoprodI.induction_on: <10 ms before, <10 ms after 🎉
• MulChar.ext': <10 ms before, <10 ms after 🎉

This golfing PR is batched under the following guidelines:

• Up to ~5 changed files per PR
• Up to ~25 changed declarations per PR
• Up to ~100 changed lines per PR

---

[github-actions]

PR summary 25ce633136

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/declarations_diff.sh <optional_commit>

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

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

---

No changes to technical debt.

You can run this locally as

./scripts/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).

[mathlib4-merge-conflict-bot]

This pull request has conflicts, please merge master and resolve them.

[jcommelin]

Thanks 🎉

bors merge

[mathlib-bors]

This PR was included in a batch that was canceled, it will be automatically retried

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• Lint style
• Build
• Post-Build Step