[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/panic.serial/display.h

@@ -6,7 +6,7 @@
 
 struct cpu_state;
 
-namespace panic {
+namespace panicking {
 
 class serial_port;
 class symbol_table;
@@ -27,4 +27,4 @@ void print_header(
 void print_callstack(serial_port &out, symbol_table &syms, frame const *fp);
 void print_cpu_state(serial_port &out, const cpu_state &regs);
 
-}
+} // namespace panicking