observer: configurable mask type

src/entt/entity/fwd.hpp

@@ -79,7 +79,7 @@ class basic_runtime_view;
 template<typename, typename, typename>
 class basic_group;
 
-template<typename>
+template<typename, typename = std::uint32_t>
 class basic_observer;
 
 template<typename>

src/entt/entity/observer.hpp

@@ -156,10 +156,11 @@ inline constexpr basic_collector<> collector{};
  * pointers.
  *
  * @tparam Registry Basic registry type.
+ * @tparam Mask Mask type.
  */
-template<typename Registry>
-class basic_observer: private basic_storage<std::uint32_t, typename Registry::entity_type> {
-    using base_type = basic_storage<std::uint32_t, typename Registry::entity_type>;
+template<typename Registry, typename Mask>
+class basic_observer: private basic_storage<Mask, typename Registry::entity_type> {
+    using base_type = basic_storage<Mask, typename Registry::entity_type>;
 
     template<typename>
     struct matcher_handler;