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.

libc/integral/cxx.inc

@@ -39,3 +39,5 @@ struct __cxx_choose_expr<false, _T, _U> {
 /* todo jart whyyyy */
 #define _Float16 __fp16
 #endif
+
+using nullptr_t = decltype(nullptr);