Change primary looping macro to 'loop' instead of 'for'.

examples/primes.dst

@@ -4,10 +4,10 @@
  "Returns a list of prime numbers less than n."
  [n]
  (def list @[])
- (for [i 2 n]
+ (loop [i :range [2 n]]
   (var isprime? true)
   (def len (length list))
-  (for [j 0 len]
+  (loop [j :range [0 len]]
    (def trial (get list j))
    (if (zero? (% i trial)) (:= isprime? false)))
   (if isprime? (array.push list i)))