Reimplement variadic if without recursion (closes hylang#842)

hy/core/bootstrap.hy

@@ -10,11 +10,14 @@
   "if with elif"
   (setv n (len args))
   (if* n
-       (if* (= n 1)
-            (get args 0)
-            `(if* ~(get args 0)
-                  ~(get args 1)
-                  (if ~@(cut args 2))))))
+       (do
+          (setv args (list args))
+          (setv else-branch (if* (% n 2) (.pop args) `None))
+          (.reverse args)
+          (reduce (fn [current-else [then-branch cond]]
+                      `(if* ~cond ~then-branch ~current-else))
+                  (zip (cut args None None 2) (cut args 1 None 2))
+                  else-branch))))
 
 (defmacro macro-error [location reason]
   "error out properly within a macro"