[kernel] Move kassert out of kutil

Continuing moving things out of kutil. The assert as implemented could
only ever work in the kernel, so remaining kutil uses of kassert have
been moved to including standard C assert instead.

Along the way, kassert was broken out into panic::panic and kassert,
and the panic.serial namespace was renamed panicking.

src/kernel/scheduler.cpp

@@ -1,9 +1,9 @@
 #include <stddef.h>
 
 #include <j6/init.h>
-#include "kutil/assert.h"
 
 #include "apic.h"
+#include "assert.h"
 #include "clock.h"
 #include "console.h"
 #include "cpu.h"