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.

Kernel/KBuffer.h

@@ -18,6 +18,7 @@
 #include <AK/Assertions.h>
 #include <AK/StringView.h>
 #include <Kernel/Memory/MemoryManager.h>
+#include <Kernel/StdLib.h> // For memcpy. FIXME: Make memcpy less expensive to access a declaration of in the Kernel.
 
 namespace Kernel {