>> -					(+ (string-width (car s))
>> -                                           (string-width (cadr s)))
>> +                                        (cl-reduce #'+ (mapcar #'string-width s))
>
> Nit: If you're calling mapcar anyway, why not simply (apply #'+ 0 ...)?

Thanks, fixed.  Now the patch also adds a new user option
'completions-detailed'.  An option separate from 'completions-format'
is needed in case when a completion function doesn't provide :affix-function,
so there is a need to fall back to the customized value of 'completions-format'.