feat: NSMul/NPow type class

[JovanGerb]

This PR is an attempt at splitting out the NSMul class for the nsmul data field in AddMonoid.

TODO: the same for ZSMul, QSMul and NNQSMul.

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[github-actions]

PR summary 044631201f

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

+ DivInvMonoid.toPow
+ NPow
+ NSMul
+ ZPow
+ ZSMul
+ instance (n : ℕ) [NeZero n] : HPow (Fin n) ℕ (Fin n)
+ instance (n : ℕ) [NeZero n] : Pow (Fin n) ℕ
+ instance (priority := low) NPow.ofPow {M : Type*} [Pow M ℕ] : NPow M := ⟨fun n x ↦ x ^ n⟩
+ instance (priority := low) ZPow.ofPow {G : Type*} [Pow G ℤ] : ZPow G
- DivInvMonoid.toZPow
- SubNegMonoid.toZSMul

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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Decrease in tech debt: (relative, absolute) = (1.24, 0.00)

Current number	Change	Type
6395	1	backward.isDefEq.respectTransparency
803	-2	backward.privateInPublic
414	-1	backward.privateInPublic.warn

Current commit fa54279527
Reference commit 044631201f

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).