Work around MSVC bug in kj::Mapper

Closes #408. Perhaps the same bug as in commit cff9ba05?

c++/src/kj/array.h

@@ -524,7 +524,7 @@ struct Mapper {
     }
     return builder.finish();
   }
-  typedef decltype(*(array).begin()) Element;
+  typedef decltype(*kj::instance<T>().begin()) Element;
 };
 
 template <typename T, size_t s>