storage: coding style/minor changes

src/entt/entity/storage.hpp

@@ -156,10 +156,10 @@ class extended_storage_iterator final {
 
 public:
     using iterator_type = It;
+    using difference_type = std::ptrdiff_t;
     using value_type = decltype(std::tuple_cat(std::make_tuple(*std::declval<It>()), std::forward_as_tuple(*std::declval<Other>()...)));
     using pointer = input_iterator_pointer<value_type>;
     using reference = value_type;
-    using difference_type = std::ptrdiff_t;
     using iterator_category = std::input_iterator_tag;
 
     constexpr extended_storage_iterator()