feat(group_theory/coset): Show that `quotient_group.left_rel` and `left_coset_equivalence` are the same thing (#8382)
feat(group_theory/coset): Show that `quotient_group.left_rel` and `left_coset_equivalence` are the same thing (#8382)





Co-authored-by: Anne Baanen <839f20e486a83a313cf46ca4094e2de98129fc5d@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib