Clean up numeric.h and split FacePositionCache from it

I also optiized FacePositionCache a bit: I removed a map
lookup and vector copy from both branches of getFacePosition.

src/face_position_cache.h

@@ -0,0 +1,44 @@
+/*
+Minetest
+Copyright (C) 2015 Nerzhul, Loic Blot <loic.blot@unix-experience.fr>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program 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 Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#ifndef FACE_POSITION_CACHE_HEADER
+#define FACE_POSITION_CACHE_HEADER
+
+#include "irr_v3d.h"
+#include "threading/mutex.h"
+#include "util/cpp11_container.h"
+
+#include <map>
+#include <vector>
+
+/*
+ * This class permits caching getFacePosition call results.
+ * This reduces CPU usage and vector calls.
+ */
+class FacePositionCache {
+public:
+	static const std::vector<v3s16> &getFacePositions(u16 d);
+
+private:
+	static const std::vector<v3s16> &generateFacePosition(u16 d);
+	static UNORDERED_MAP<u16, std::vector<v3s16> > cache;
+	static Mutex cache_mutex;
+};
+
+#endif