Improve random bignum generation

* src/bignum.c (get_random_limb, get_random_limb_lim)
(get_random_bignum): New functions, for more-efficient
generation of random bignums without using Frem etc.
* src/fns.c (get_random_fixnum): New function.
(Frandom): Use it, and get_random_bignum.
Be consistent about signalling nonpositive integer arguments;
since zero is invalid, Qnatnump is not quite right here.
* src/sysdep.c (get_random_ulong): New function.

src/sysdep.c

@@ -2200,6 +2200,16 @@ get_random (void)
   return val & INTMASK;
 }
 
+/* Return a random unsigned long.  */
+unsigned long int
+get_random_ulong (void)
+{
+  unsigned long int r = 0;
+  for (int i = 0; i < (ULONG_WIDTH + RAND_BITS - 1) / RAND_BITS; i++)
+    r = random () ^ (r << RAND_BITS) ^ (r >> (ULONG_WIDTH - RAND_BITS));
+  return r;
+}
+
 #ifndef HAVE_SNPRINTF
 /* Approximate snprintf as best we can on ancient hosts that lack it.  */
 int