Pairs !- tuples, so add some better Meta support

I usually use `tuple` for all product types, but some STL bits
use pair, which is a distinct type.  So here's some helpful
meta helpers for them.

Meta/pair_decay.h

@@ -0,0 +1,36 @@
+static_assert( __cplusplus > 201700, "C++17 Required" );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <type_traits>
+
+#include <Alepha/Stud/type_traits.h>
+
+namespace Alepha::Hydrogen::Meta
+{
+	inline namespace exports { inline namespace type_traits {} }
+
+	namespace detail::type_traits::pair_decay
+	{
+		inline namespace exports
+		{
+			template< typename T > struct pair_decay;
+
+			template< typename First, typename Second >
+			struct pair_decay< std::pair< First, Second > >
+			{
+				using type= std::pair< Stud::decay_t< First >, Stud::decay_t< Second > >;
+			};
+
+			template< typename T >
+			using pair_decay_t= typename pair_decay< T >::type;
+		}
+	}
+
+	namespace exports::type_traits
+	{
+		using namespace detail::type_traits::pair_decay::exports;
+	}
+}