Everywhere: Remove string.h include from AK/Traits.h and resolve fallout

A lot of places were relying on AK/Traits.h to give it strnlen, memcmp,
memcpy and other related declarations.

In the quest to remove inclusion of LibC headers from Kernel files, deal
with all the fallout of this included-everywhere header including less
things.

Userland/Libraries/LibELF/Image.cpp

@@ -14,7 +14,12 @@
 #include <Kernel/API/serenity_limits.h>
 #include <LibELF/Image.h>
 #include <LibELF/Validation.h>
-#include <limits.h>
+
+#ifdef KERNEL
+#    include <Kernel/StdLib.h>
+#else
+#    include <string.h>
+#endif
 
 namespace ELF {