Make IO a static class instead of a singleton.

test/variable/local_in_middle_of_block.wren

@@ -1,13 +1,13 @@
 class Foo {
   bar {
     var a = "a"
-    io.write(a) // expect: a
+    IO.write(a) // expect: a
     var b = a + " b"
-    io.write(b) // expect: a b
+    IO.write(b) // expect: a b
     var c = a + " c"
-    io.write(c) // expect: a c
+    IO.write(c) // expect: a c
     var d = b + " d"
-    io.write(d) // expect: a b d
+    IO.write(d) // expect: a b d
   }
 }