[Merged by Bors] - feat(LinearAlgebra/Matrix/Charpoly/Eigs): generalize to CommRings

[SnirBroshi]

Also adds r ∈ spectrum R B ↔ ¬IsUnit (B.charpoly.eval r) which is the ring version of r ∈ spectrum K A ↔ A.charpoly.IsRoot r.

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The variables above define A over a Field and B over a CommRing, switching A to B generalizes.

[github-actions]

PR summary ad4f90556a

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

+ mem_spectrum_iff_not_isUnit_eval_charpoly

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.

[b-mehta]

This looks good to me but it's not my area so

maintainer merge

[github-actions]

🚀 Pull request has been placed on the maintainer queue by b-mehta.

[riccardobrasca]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

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