xml, cbor, csv, protobuf: add Sort.kinded' helper
Factor [kinded] through [kinded' ~kind s = if kind = "" then s else
kind ^ " " ^ s], matching the shape of json's and toml's Sort
modules. Consistent API across every encoding library's Sort: every
module exposes [to_string], [pp], [or_kind], [kinded'], [kinded].
protobuf previously lacked [or_kind] and [kinded] entirely; add them
alongside [kinded'].