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/list/remove_at.wren

@@ -1,32 +1,32 @@
 var a = [1, 2, 3]
 a.removeAt(0)
-io.write(a) // expect: [2, 3]
+IO.write(a) // expect: [2, 3]
 
 var b = [1, 2, 3]
 b.removeAt(1)
-io.write(b) // expect: [1, 3]
+IO.write(b) // expect: [1, 3]
 
 var c = [1, 2, 3]
 c.removeAt(2)
-io.write(c) // expect: [1, 2]
+IO.write(c) // expect: [1, 2]
 
 // Index backwards from end.
 var d = [1, 2, 3]
 d.removeAt(-3)
-io.write(d) // expect: [2, 3]
+IO.write(d) // expect: [2, 3]
 
 var e = [1, 2, 3]
 e.removeAt(-2)
-io.write(e) // expect: [1, 3]
+IO.write(e) // expect: [1, 3]
 
 var f = [1, 2, 3]
 f.removeAt(-1)
-io.write(f) // expect: [1, 2]
+IO.write(f) // expect: [1, 2]
 
 // Out of bounds.
 // TODO: Signal error in better way.
-io.write([1, 2, 3].removeAt(3)) // expect: null
-io.write([1, 2, 3].removeAt(-4)) // expect: null
+IO.write([1, 2, 3].removeAt(3)) // expect: null
+IO.write([1, 2, 3].removeAt(-4)) // expect: null
 
 // Return the removed value.
-io.write([3, 4, 5].removeAt(1)) // expect: 4
+IO.write([3, 4, 5].removeAt(1)) // expect: 4