Add good old factorial.

programs/factorial.lily

@@ -0,0 +1,12 @@
+defn if c t e = {
+    case c of {
+        True -> { t }
+        False -> { e }
+    }
+}
+
+defn fact n = {
+    if (eq n 0) 1 (n * fact (n - 1))
+}
+
+defn main = { fact 6 }