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/field/default_to_null.wren

@@ -1,5 +1,5 @@
 class Foo {
-  write { io.write(_field) }
+  write { IO.write(_field) }
 }
 
 (new Foo).write // expect: null

test/field/multiple.wren

@@ -7,11 +7,11 @@ class Foo {
     _e = e
   }
   write {
-    io.write(_a)
-    io.write(_b)
-    io.write(_c)
-    io.write(_d)
-    io.write(_e)
+    IO.write(_a)
+    IO.write(_b)
+    IO.write(_c)
+    IO.write(_d)
+    IO.write(_e)
   }
 }
 

test/field/object_reference.wren

@@ -11,7 +11,7 @@ class Node {
       _left.write
     }
 
-    io.write(_value)
+    IO.write(_value)
 
     if (_right is Node) {
       _right.write

test/field/use_before_set.wren

@@ -1,5 +1,5 @@
 class Foo {
-  write { io.write(_field) } // Compile a use of the field...
+  write { IO.write(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }