Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/LibC/stddef.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <sys/cdefs.h>
+#include <sys/types.h>
+
+#ifdef __cplusplus
+#    define NULL nullptr
+#else
+#    define NULL ((void*)0)
+#endif
+
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+typedef __SIZE_TYPE__ size_t;