refactor: convert IO from singleton instance to static class with metaclass native methods

Replace the global `io` singleton object with a static `IO` class, moving the `write` native from the instance to the metaclass. Update all benchmark, example, and test files to use `IO.write()` instead of `io.write()`.

test/class/equality.wren

@@ -1,13 +1,13 @@
-io.write(Num == Num)  // expect: true
-io.write(Num == Bool) // expect: false
+IO.write(Num == Num)  // expect: true
+IO.write(Num == Bool) // expect: false
 
 // Not equal to other types.
-io.write(Num == 123)  // expect: false
-io.write(Num == true) // expect: false
+IO.write(Num == 123)  // expect: false
+IO.write(Num == true) // expect: false
 
-io.write(Num != Num)  // expect: false
-io.write(Num != Bool) // expect: true
+IO.write(Num != Num)  // expect: false
+IO.write(Num != Bool) // expect: true
 
 // Not equal to other types.
-io.write(Num != 123)  // expect: true
-io.write(Num != true) // expect: true
+IO.write(Num != 123)  // expect: true
+IO.write(Num != true) // expect: true

test/class/name_inside_body.wren

@@ -1,10 +1,10 @@
 class Foo {
   static sayName {
-    io.write(Foo.toString)
+    IO.write(Foo.toString)
   }
 
   sayName {
-    io.write(Foo.toString)
+    IO.write(Foo.toString)
   }
 
   static toString { return "Foo!" }

test/class/new.wren

@@ -1,6 +1,6 @@
 class Foo {}
 
 var foo = new Foo
-io.write(foo is Foo) // expect: true
+IO.write(foo is Foo) // expect: true
 
 // TODO: Test precedence and grammar of what follows "new".