Make IO a static class instead of a singleton.

test/this/closure.wren

@@ -9,4 +9,4 @@ class Foo {
 }
 
 var closure = (new Foo).getClosure
-io.write(closure.call) // expect: Foo
+IO.write(closure.call) // expect: Foo

test/this/nested_class.wren

@@ -1,13 +1,13 @@
 class Outer {
   method {
-    io.write(this.toString) // expect: Outer
+    IO.write(this.toString) // expect: Outer
 
     fn {
-      io.write(this.toString) // expect: Outer
+      IO.write(this.toString) // expect: Outer
 
       class Inner {
         method {
-          io.write(this.toString) // expect: Inner
+          IO.write(this.toString) // expect: Inner
         }
         toString { return "Inner" }
       }

test/this/nested_closure.wren

@@ -13,4 +13,4 @@ class Foo {
 }
 
 var closure = (new Foo).getClosure
-io.write(closure.call.call.call) // expect: Foo
+IO.write(closure.call.call.call) // expect: Foo

test/this/this_in_method.wren

@@ -3,4 +3,4 @@ class Foo {
   baz { return "baz" }
 }
 
-io.write((new Foo).bar.baz) // expect: baz
+IO.write((new Foo).bar.baz) // expect: baz