Add additional type to allow for types themselves to be declared deprecated (through using statements and the like).

sol/deprecate.hpp

@@ -32,4 +32,13 @@
     #endif // compilers
 #endif // SOL_DEPRECATED
 
+namespace sol {
+namespace detail {
+   template <typename T>
+   struct SOL_DEPRECATED deprecate_type {
+       using type = T;
+   };
+} // detail
+} // sol
+
 #endif // SOL_DEPRECATE_HPP