refactor: move tuple_cat_t to concepts.h

include/iterator/concepts.h

@@ -117,4 +117,7 @@ concept AssocRange = Assoc<V> && (std::tuple_size_v<V> == 2) &&
 
 template <typename C>
 using iterator_t = decltype(std::begin(std::declval<C &>()));
+
+template <typename... Ts>
+using tuple_cat_t = decltype(std::tuple_cat(std::declval<Ts>()...));
 }

include/iterator/recursive_iterator.h

@@ -19,9 +19,6 @@
 #include <iterator/detail/macro.h>
 
 namespace iterator {
-template <typename... Ts>
-using tuple_cat_t = decltype(std::tuple_cat(std::declval<Ts>()...));
-
 template <typename It, typename MaxDepth, size_t N = 0,
           typename V = std::iter_value_t<It>>
 struct tuple_expander {