Add type alias for underlying type of vk::UniqueHandle (#326)

* Add type alias for underlying type of vk::UniqueHandle

* Type alias to UniqueHandle

* Unique Handle Type alias moved

vulkan/vulkan.hpp

@@ -509,6 +509,9 @@ namespace VULKAN_HPP_NAMESPACE
   private:
     using Deleter = typename UniqueHandleTraits<Type,Dispatch>::deleter;
   public:
+
+    using element_type = Type;
+
     explicit UniqueHandle( Type const& value = Type(), Deleter const& deleter = Deleter() )
       : Deleter( deleter)
       , m_value( value )