Make IO a static class instead of a singleton.

test/super/call_other_method.wren

@@ -1,12 +1,12 @@
 class Base {
   foo {
-    io.write("Base.foo")
+    IO.write("Base.foo")
   }
 }
 
 class Derived is Base {
   bar {
-    io.write("Derived.bar")
+    IO.write("Derived.bar")
     super.foo
   }
 }

test/super/call_same_method.wren

@@ -1,12 +1,12 @@
 class Base {
   foo {
-    io.write("Base.foo")
+    IO.write("Base.foo")
   }
 }
 
 class Derived is Base {
   foo {
-    io.write("Derived.foo")
+    IO.write("Derived.foo")
     super.foo
   }
 }

test/super/indirectly_inherited.wren

@@ -1,6 +1,6 @@
 class A {
   foo {
-    io.write("A.foo")
+    IO.write("A.foo")
   }
 }
 
@@ -8,7 +8,7 @@ class B is A {}
 
 class C is B {
   foo {
-    io.write("C.foo")
+    IO.write("C.foo")
     super.foo
   }
 }