[variant] Only store trivial data types directly

O/w certain types (for instance FieldVectors) could not be stored.

dune/subgrid/common/variant.hh

@@ -138,7 +138,7 @@ namespace Impl {
     }
 
     private:
-    TypeStorage_<Head_, std::is_literal_type<Head_>::value> head_;
+    TypeStorage_<Head_, std::is_trivial<Head_>::value> head_;
     variant_union_<Tail_...> tail_;
   };