stl: Update headers

type_traits: Add is_integral
optional: Add stub optional class
iterator: Add default_sentinel_t & default_sentinel
concepts: Add concepts header + integral concept

include_std/optional

@@ -0,0 +1,26 @@
+/*
+	This file is part of Fennix Kernel.
+
+	Fennix Kernel is free software: you can redistribute it and/or
+	modify it under the terms of the GNU General Public License as
+	published by the Free Software Foundation, either version 3 of
+	the License, or (at your option) any later version.
+
+	Fennix Kernel is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with Fennix Kernel. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#pragma once
+
+#include <concepts>
+
+namespace std
+{
+	template <class T>
+	class optional;
+}