feat: product of Coxeter matrices

[vihdzp]

The Coxeter matrix A * B is the block matrix !![A, 2; 2, B].

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I've used multiplication to denote this under the logic that the product of Coxeter groups has a Coxeter matrix given by this construction. Perhaps this isn't desirable, considering it doesn't match matrix multiplication. I'm open to suggestions.

[github-actions]

PR summary fb131a4c00

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

+ instance : CoeFun (CoxeterMatrix B) fun _ ↦ Matrix B B ℕ := ⟨M⟩
+ instance : HMul (CoxeterMatrix B) (CoxeterMatrix B') (CoxeterMatrix (B ⊕ B'))
- instance : CoeFun (CoxeterMatrix B) fun _ ↦ (Matrix B B ℕ) := ⟨M⟩

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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No changes to technical debt.

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).

[tb65536]

Yeah, maybe this should just be a def with API instead?

[vihdzp]

a) What should I call this?
b) What API should I put on this? This is mostly intended as a way to prove products of Coxeter groups are Coxeter, should I hold off on adding it before I prove that theorem?

[tb65536]

Yeah, maybe wait until we have that to look at.

[mathlib-bors]

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