mathlib3

5f0dbf65 - feat(algebra/group/conj): `is_conj` for `monoid`s, `is_conj.setoid`, and `conj_classes` (#6896)

feat(algebra/group/conj): `is_conj` for `monoid`s, `is_conj.setoid`, and `conj_classes` (#6896) Refactors `is_conj` to work in a `monoid` Defines `is_conj.setoid` and its quotient type, `conj_classes` Co-authored-by: Johan Commelin <johan@commelin.net>