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/number/minus.wren

@@ -1,10 +1,10 @@
 // Infix.
-io.write(5 - 3)      // expect: 2
-io.write(3.1 - 0.24) // expect: 2.86
-io.write(3 - 2 - 1)  // expect: 0
+IO.write(5 - 3)      // expect: 2
+IO.write(3.1 - 0.24) // expect: 2.86
+IO.write(3 - 2 - 1)  // expect: 0
 
 // Unary negation.
 var a = 3
-io.write(-a) // expect: -3
+IO.write(-a) // expect: -3
 
 // TODO: Unsupported RHS types.