Add native UUID generator.

While not necessary, since we barely use UUID:s, it's still WAY faster,
and nice to not depend on the system providing `uuidgen'.

For comparison, number of UUID:s generated by different settups in 1
second:

   65.000 :: guile native
      700 :: guile shell-out
3.500.000 :: c native
  100.000 :: libuuid - guile bindings
  650.000 :: libuuid

Tests where run with either
    (call-with-time-limit
      1
      (lambda ()
	(let loop ()
	 (uuid)
	 (set! count (1+ count))
	 (loop)))
      (lambda _ (display count)))
or
    volatile unsigned long long count = 0;
    sigaction(SIGALARM, ...);
    alarm(1);
    for (;;) {
	uuid();
	count++;
    }

(let ((count 0))
  (call-with-time-limit
   1
   (lambda ()
     (let loop ()
       ((@ (hnh util) uuidgen))
       (set! count (1+ count))
       (loop)))
   (lambda _ (format #t "exec count = ~:d~%" count))))

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