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/inheritance/is.wren

@@ -5,12 +5,12 @@ var a = new A
 var b = new B
 var c = new C
 
-io.write(a is A) // expect: true
-io.write(a is B) // expect: false
-io.write(a is C) // expect: false
-io.write(b is A) // expect: true
-io.write(b is B) // expect: true
-io.write(b is C) // expect: false
-io.write(c is A) // expect: true
-io.write(c is B) // expect: true
-io.write(c is C) // expect: true
+IO.write(a is A) // expect: true
+IO.write(a is B) // expect: false
+IO.write(a is C) // expect: false
+IO.write(b is A) // expect: true
+IO.write(b is B) // expect: true
+IO.write(b is C) // expect: false
+IO.write(c is A) // expect: true
+IO.write(c is B) // expect: true
+IO.write(c is C) // expect: true