chore(Algebra/Polynomial/Module): workaround for backward.inferInstanceAs

[Vierkantor]

This PR, like #38990, works around a backward.inferInstanceAs compatibility flag introduced by identifying PolynomialModule with Finsupp in our definitions. We introduce a new dsimp lemma funLike_eq that transfers the FunLike instances, and now we can use PolynomialModule's FunLike instance, instead of the custom CoeFun instance.

This is not a great approach, but it seems the least painful for the short term. The alternative would be to strictly enforce the defeq barrier between PolynomialModule and Finsupp, which would mean a substantial rewrite of this corner of Mathlib. We can't make PolynomialModule an @[implicit_reducible], because we need different multiplication on it than Finsupp has.

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[github-actions]

PR summary 8885ca9642

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

+ funLike_eq

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.