hacky bugfix: support floats in nth

Needed because we don't yet have a primitive in the shell to truncate/round
non-integers to integers.

Before:
  (nth (/ 31 10)    # we don't have float literals yet
       '(1 2 3 4))
  => NULL

..with an unpleasant abort likely later on.

Really the correct thing to do is ensure none of my primitives ever returns
NULL. Start with car/cdr.

shell/data.limg

@@ -22,7 +22,7 @@
     0
     (+ 1 (len (cdr l)))))])
     (nth . [(def (nth n xs)
-  (if (<= n 0)
+  (if (< n 1)
     (car xs)
     (nth (- n 1) (cdr xs))))])
     (map1 . [(def (map1 f xs)