feat: A linter for instances that overlap on data

[JovanGerb]

This PR defines a linter for overlapping instances in declarations. We want to warn users about this, because it leads to local instance diamonds.

TODO

• expand documentation
• Ideally, the linter would be active at all times, included in the standard linter set, and it should also be able to fire on partial declarations, since it only works with the type of the declaration.

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[github-actions]

PR summary 959c85291a

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Tactic	1
Mathlib.Tactic.Linter.OverlappingInstances (new file)	28

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Declarations diff

+ findDuplicateDataInstances
+ overlappingInstances

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>

The doc-module for script/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/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).

[euprunin]

"conpute return" should be "compute and return"?