Make IO a static class instead of a singleton.

test/setter/associativity.wren

@@ -13,6 +13,6 @@ var c = new Foo("c")
 
 // Assignment is right-associative.
 a.bar = b.bar = c.bar = "d"
-io.write(a.toString) // expect: d
-io.write(b.toString) // expect: d
-io.write(c.toString) // expect: d
+IO.write(a.toString) // expect: d
+IO.write(b.toString) // expect: d
+IO.write(c.toString) // expect: d

test/setter/instance.wren

@@ -1,6 +1,6 @@
 class Foo {
   bar = value {
-    io.write(value)
+    IO.write(value)
   }
 }
 

test/setter/result.wren

@@ -3,4 +3,4 @@ class Foo {
 }
 
 var foo = new Foo
-io.write(foo.bar = "value") // expect: result
+IO.write(foo.bar = "value") // expect: result

test/setter/same_name_as_method.wren

@@ -1,6 +1,6 @@
 class Foo {
-  bar = value { io.write("set") }
-  bar { io.write("get") }
+  bar = value { IO.write("set") }
+  bar { IO.write("get") }
 }
 
 var foo = new Foo

test/setter/static.wren

@@ -1,6 +1,6 @@
 class Foo {
   static bar = value {
-    io.write(value)
+    IO.write(value)
   }
 }