Made the fib benchmark a bit faster

This way fib.wren was consistently about 6% faster for me.

benchmark/fib.wren

@@ -1,10 +1,12 @@
-var fib = new Fn {|n|
+class Fib {
-  if (n < 2) return n
+  static get(n) {
-  return fib.call(n - 1) + fib.call(n - 2)
+    if (n < 2) return n
+    return get(n - 1) + get(n - 2)
+  }
 }
 
 var start = IO.clock
 for (i in 1..5) {
-  IO.print(fib.call(28))
+  IO.print(Fib.get(28))
 }
 IO.print("elapsed: ", IO.clock - start)