Add a 'tail recursive' fibonacci program

programs/fib.lily

@@ -0,0 +1,14 @@
+defn if c t e = {
+    case c of {
+        True -> { t }
+        False -> { e }
+    }
+}
+
+defn fibtr a b n = {
+    if (eq n 0) a (fibtr b (a+b) (n-1))
+}
+
+defn fib n = { fibtr 1 1 n }
+
+defn main = { fib 40 }