Theorem monoid_hom.id_comp

refactor(algebra/*): add new `mul_one_class` and `add_zero_class` for non-associative monoids (#6865) …

chore(*/hom,equiv): Split `monoid_hom` into more fundamental structures, and reuse them elsewhere (#4423) …

refactor(group_theory/submonoid): review API (#2173) …