CTL: utility.h, use ctl::swap in string (#1227)

* Add ctl utility.h

Implements forward, move, swap, and declval. This commit also adds a def
for nullptr_t to cxx.inc. We need it now because the CTL headers stopped
including anything from libc++, so we no longer get their basic types.

* Use ctl::swap in string

The STL spec says that swap is located in the string_view header anyawy.
Performance-wise this is a noop, but it’s slightly cleaner.

ctl/string_view.h

@@ -2,6 +2,7 @@
 // vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
 #ifndef COSMOPOLITAN_CTL_STRINGVIEW_H_
 #define COSMOPOLITAN_CTL_STRINGVIEW_H_
+#include "utility.h"
 
 namespace ctl {