parse/enum: use references for range binding iteration to avoid copies

parse/enum.hpp

@@ -143,7 +143,7 @@ namespace cruft::parse::enumeration {
                 static_assert (std::is_integral_v<ValueT>);
 
                 if constexpr (std::is_same_v<ValueT, underlying_type>) {
-                    for (auto const [key,val]: m_cache) {
+                    for (auto const &[key,val]: m_cache) {
                         if (equal (key, str)) {
                             str = str.consume (key.size ());
                             // We must cast the value here so that enum