mathlib

455393d3 - refactor(group_theory/{submonoid, subsemigroup}/{center, centralizer}): move set.center and set.centralizer into subsemigroup (#13903)

refactor(group_theory/{submonoid, subsemigroup}/{center, centralizer}): move set.center and set.centralizer into subsemigroup (#13903) This moves `set.center` and `set.centralizer` (the center and centralizers for a magma) into `group_theory/subsemigroup/{center, centralizer}` so that we can define the center and centralizers for semigroups in #13627.