[Merged by Bors] - feat(GroupTheory/Congruences): add the equiv between congruence relations on a group and its normal subgroup

[xroblot]

Let G be a group, we prove

{ N : Subgroup G // N.Normal } ≃o Con G

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[github-actions]

PR summary 16ac32e820

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

+ _root_.Con.mem_subgroup_iff
+ _root_.Con.subgroup
+ _root_.Con.subgroup_quotientGroupCon
+ _root_.Subgroup.orderIsoCon
+ con_ker_eq_conKer
+ con_le_iff
+ con_mono
+ con_subgroup
+ instance (c : Con G) : c.subgroup.Normal

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).

[eric-wieser]

This is basically what I was hoping for, thanks!

[eric-wieser]

bors d+

Thanks!

[mathlib-bors]

✌️ xroblot can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[xroblot]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

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