Add an instance {add_,}sub{monoid,group}_class S M → has_coe_t S ({add_,}sub{monoid,group} M)

[Vierkantor]

We are missing instances add_submonoid_class S M → has_coe S (add_submonoid M) (probably has_coe_t now that I think of it), so we can just use coe here.

Similar coercions should also be added for subgroup and other subobjects.