Make IO a static class instead of a singleton.

test/list/subscript_setter.wren

@@ -4,13 +4,13 @@
   list[0] = 5
   list[1] = 6
   list[2] = 7
-  io.write(list) // expect: [5, 6, 7]
+  IO.write(list) // expect: [5, 6, 7]
 }
 
 // Returns right-hand side.
 {
   var list = [1, 2, 3]
-  io.write(list[1] = 5) // expect: 5
+  IO.write(list[1] = 5) // expect: 5
 }
 
 // Negative indices.
@@ -19,7 +19,7 @@
   list[-1] = 5
   list[-2] = 6
   list[-3] = 7
-  io.write(list) // expect: [7, 6, 5]
+  IO.write(list) // expect: [7, 6, 5]
 }
 
 // TODO: Handle out of range.