Make IO a static class instead of a singleton.

benchmark/fib.wren

@@ -9,7 +9,7 @@ var fib = fn(n) {
 var start = OS.clock
 var i = 0
 while (i < 5) {
-  io.write(fib.call(28))
+  IO.write(fib.call(28))
   i = i + 1
 }
-io.write("elapsed: " + (OS.clock - start).toString)
+IO.write("elapsed: " + (OS.clock - start).toString)