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/is.wren

@@ -1,32 +1,32 @@
-io.write(Num is Class) // expect: true
-io.write(true is Bool) // expect: true
-io.write((fn 1) is Function) // expect: true
-io.write(123 is Num) // expect: true
-io.write(null is Null) // expect: true
-io.write("s" is String) // expect: true
+IO.write(Num is Class) // expect: true
+IO.write(true is Bool) // expect: true
+IO.write((fn 1) is Function) // expect: true
+IO.write(123 is Num) // expect: true
+IO.write(null is Null) // expect: true
+IO.write("s" is String) // expect: true
 
-io.write(Num is Bool) // expect: false
-io.write(null is Class) // expect: false
-io.write(true is Function) // expect: false
-io.write((fn 1) is Num) // expect: false
-io.write("s" is Null) // expect: false
-io.write(123 is String) // expect: false
+IO.write(Num is Bool) // expect: false
+IO.write(null is Class) // expect: false
+IO.write(true is Function) // expect: false
+IO.write((fn 1) is Num) // expect: false
+IO.write("s" is Null) // expect: false
+IO.write(123 is String) // expect: false
 
 // Everything extends Object.
-io.write(Num is Object) // expect: true
-io.write(null is Object) // expect: true
-io.write(true is Object) // expect: true
-io.write((fn 1) is Object) // expect: true
-io.write("s" is Object) // expect: true
-io.write(123 is Object) // expect: true
+IO.write(Num is Object) // expect: true
+IO.write(null is Object) // expect: true
+IO.write(true is Object) // expect: true
+IO.write((fn 1) is Object) // expect: true
+IO.write("s" is Object) // expect: true
+IO.write(123 is Object) // expect: true
 
 // Classes extend Class.
-io.write(Num is Class) // expect: true
-io.write(null is Class) // expect: false
-io.write(true is Class) // expect: false
-io.write((fn 1) is Class) // expect: false
-io.write("s" is Class) // expect: false
-io.write(123 is Class) // expect: false
+IO.write(Num is Class) // expect: true
+IO.write(null is Class) // expect: false
+IO.write(true is Class) // expect: false
+IO.write((fn 1) is Class) // expect: false
+IO.write("s" is Class) // expect: false
+IO.write(123 is Class) // expect: false
 
 // TODO: Non-class on RHS.
 // TODO: Precedence and associativity.