Started to rewrite traits and recursive traits

traits are shorter and easier to identify by using tags in a variadic
template

recursive traits can be automatically derived, which makes it much
easier to add new traits to be handed through the expression tree.

include/sqlpp11/default_value.h

@@ -33,9 +33,8 @@ namespace sqlpp
 {
 	struct default_value_t
 	{
-		static constexpr bool _is_expression = true;
-		using _value_type = no_value_t;
-		using _table_set = ::sqlpp::detail::type_set<>;
+		using _traits = make_traits_t<no_value_t, tag::expression>;
+		using _recursive_traits = make_recursive_traits_t<>;
 
 		static constexpr bool _is_trivial() { return false; }
 	};