[Merged by Bors] - chore: Move MulAction on Opposite, Pi, Prod, Sum, Sigma, Units

[YaelDillies]

... and add assert_not_exists MonoidWithZero everywhere.

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[github-actions]

PR summary 32ffc0d2bb

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Algebra.GroupPower.IterateHom	314	310	-4 (-1.27%)
Mathlib.Algebra.Group.Aut	350	347	-3 (-0.86%)
Mathlib.Algebra.Group.NatPowAssoc	344	343	-1 (-0.29%)
Mathlib.Algebra.Group.PNatPowAssoc	403	402	-1 (-0.25%)
Mathlib.Algebra.BigOperators.Pi	508	507	-1 (-0.20%)
Mathlib.Algebra.BigOperators.Fin	544	543	-1 (-0.18%)

Import changes for all files

Files	Import difference
Too many changes (2843)!

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Declarations diff

+ extend_smul
+ instance [Monoid M] [SMul M α] : SMul Mˣ α where smul m a := (m : M) • a
++-- smul_def
+-++-- smulCommClass
- Function.extend_smul

You can run this locally as follows

## summary with just the declaration names:
./scripts/no_lost_declarations.sh short <optional_commit>

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

[kim-em]

bors merge

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[riccardobrasca]

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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