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()`.

benchmark/binary_trees.wren

@@ -26,7 +26,7 @@ var stretchDepth = maxDepth + 1
 
 var start = OS.clock
 
-io.write("stretch tree of depth " + stretchDepth.toString + " check: " +
+IO.write("stretch tree of depth " + stretchDepth.toString + " check: " +
     new Tree(0, stretchDepth).check.toString)
 
 var longLivedTree = new Tree(0, maxDepth)
@@ -48,13 +48,13 @@ while (depth < stretchDepth) {
     i = i + 1
   }
 
-  io.write((iterations * 2).toString + " trees of depth " + depth.toString +
+  IO.write((iterations * 2).toString + " trees of depth " + depth.toString +
       " check: " + check.toString)
   iterations = iterations / 4
   depth = depth + 2
 }
 
-io.write("long lived tree of depth " + maxDepth.toString + " check: " +
+IO.write("long lived tree of depth " + maxDepth.toString + " check: " +
     longLivedTree.check.toString)
 
-io.write("elapsed: " + (OS.clock - start).toString)
+IO.write("elapsed: " + (OS.clock - start).toString)

benchmark/fib.wren

@@ -9,7 +9,7 @@ var fib = fn(n) {
 var start = OS.clock
 var i = 0
 while (i < 5) {
-  io.write(fib.call(28))
+  IO.write(fib.call(28))
   i = i + 1
 }
-io.write("elapsed: " + (OS.clock - start).toString)
+IO.write("elapsed: " + (OS.clock - start).toString)

benchmark/method_call.wren

@@ -39,7 +39,7 @@ while (i < n) {
   i = i + 1
 }
 
-io.write(toggle.value)
+IO.write(toggle.value)
 
 val = true
 var ntoggle = new NthToggle(val, 3)
@@ -50,6 +50,6 @@ while (i < n) {
   i = i + 1
 }
 
-io.write(ntoggle.value)
+IO.write(ntoggle.value)
 
-io.write("elapsed: " + (OS.clock - start).toString)
+IO.write("elapsed: " + (OS.clock - start).toString)