mathlib

9d40a598 - feat(group_theory,linear_algebra): third isomorphism theorem for groups and modules (#8203)

feat(group_theory,linear_algebra): third isomorphism theorem for groups and modules (#8203) This PR proves the third isomorphism theorem for (additive) groups and modules, and also adds a few `simp` lemmas that I needed. Co-authored-by: Anne Baanen <Vierkantor@users.noreply.github.com>