tuple/value: map should return return a forwarding tuple

This allows us to maintain references, rather than provoke copies.

tuple/value.hpp

@@ -144,7 +144,7 @@ namespace cruft::tuple::value {
         auto
         map (std::index_sequence<Indices...>, FuncT &&func, ArgT &&arg)
         {
-            return std::tuple (
+            return std::forward_as_tuple (
                 std::invoke (func, std::get<Indices> (arg))...
             );
         }