refactor: make sentinal_iterator::sentinal itself instead of a sentinal_type

include/iterator/sentinal_iterator.h

@@ -16,8 +16,7 @@ namespace iterator {
 template <typename It>
 class sentinal_iterator : public facade<sentinal_iterator<It>> {
 public:
-  using sentinal_type = typename It::sentinal_type;
-  static constexpr sentinal_type sentinal{};
+  static inline const sentinal_iterator<It> sentinal{};
 
 public:
   sentinal_iterator() = default;