[Merged by Bors] - feat(GroupTheory/Coset): add equivalences of fibers of monoid homomorphisms

[Multramate]

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[github-actions]

PR summary 1a17230b9e

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

+ fiberEquiv
+ fiberEquivKer
+ fiberEquivKerOfSurjective
+ fiberEquivKer_apply
+ fiberEquivKer_symm_apply
+ fiberEquivOfSurjective
+ fiberEquiv_apply
+ fiberEquiv_symm_apply

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>

[kim-em]

bors d+

[mathlib-bors]

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[Multramate]

bors r+

[mathlib-bors]

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[mathlib-bors]

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