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/constructor/default.wren

@@ -4,5 +4,5 @@ class Foo {
 
 // Classes inherit the argument-less "new" one by default.
 var foo = new Foo
-io.write(foo is Foo) // expect: true
-io.write(foo.toString) // expect: Foo
+IO.write(foo is Foo) // expect: true
+IO.write(foo.toString) // expect: Foo

test/constructor/infix_class_expression.wren

@@ -2,6 +2,6 @@ class Foo {
   + other { return "Foo " + other }
 }
 
-io.write(new Foo + "value") // expect: Foo value
+IO.write(new Foo + "value") // expect: Foo value
 
 // TODO: Other expressions following a constructor, like new Foo.bar("arg").

test/constructor/new.wren

@@ -1,7 +1,7 @@
 class Foo {
-  new { io.write("zero") }
-  new(a) { io.write(a) }
-  new(a, b) { io.write(a + b) }
+  new { IO.write("zero") }
+  new(a) { IO.write(a) }
+  new(a, b) { IO.write(a + b) }
 
   toString { return "Foo" }
 }
@@ -13,5 +13,5 @@ new Foo("one", "two") // expect: onetwo
 
 // Returns the new instance.
 var foo = new Foo // expect: zero
-io.write(foo is Foo) // expect: true
-io.write(foo.toString) // expect: Foo
+IO.write(foo is Foo) // expect: true
+IO.write(foo.toString) // expect: Foo

test/constructor/superclass.wren

@@ -1,6 +1,6 @@
 class A {
   new(arg) {
-    io.write("new A " + arg)
+    IO.write("new A " + arg)
     _field = arg
   }
 
@@ -10,7 +10,7 @@ class A {
 class B is A {
   new(arg1, arg2) {
     super(arg2)
-    io.write("new B " + arg1)
+    IO.write("new B " + arg1)
     _field = arg1
   }
 
@@ -20,7 +20,7 @@ class B is A {
 class C is B {
   new {
     super("one", "two")
-    io.write("new C")
+    IO.write("new C")
     _field = "c"
   }
 
@@ -31,10 +31,10 @@ var c = new C
 // expect: new A two
 // expect: new B one
 // expect: new C
-io.write(c is A) // expect: true
-io.write(c is B) // expect: true
-io.write(c is C) // expect: true
+IO.write(c is A) // expect: true
+IO.write(c is B) // expect: true
+IO.write(c is C) // expect: true
 
-io.write(c.aField) // expect: two
-io.write(c.bField) // expect: one
-io.write(c.cField) // expect: c
+IO.write(c.aField) // expect: two
+IO.write(c.bField) // expect: one
+IO.write(c.cField) // expect: c