chore(algebra/group/hom): add `is_monoid_hom.of_mul`, use it (#1225)
chore(algebra/group/hom): add `is_monoid_hom.of_mul`, use it (#1225)

* Let `to_additive` generate `is_add_monoid_hom.map_add`

* Converting `is_mul_hom` into `is_monoid_hom` doesn't require `α` to be a group

* Simplify the proof of `is_add_group_hom.map_sub`

Avoid `simp` (without `only`)

leanprover-community/mathlib,leanprover-community/mathlib