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/comments/block.wren

@@ -1,5 +1,5 @@
 // In middle of line.
-io.write/* ... */(/* */"ok"/* */) // expect: ok
+IO.write/* ... */(/* */"ok"/* */) // expect: ok
 
 // Nested.
-io.write(/* in /* nest */ out */"ok") // expect: ok
+IO.write(/* in /* nest */ out */"ok") // expect: ok

test/comments/block_at_eof.wren

@@ -1,2 +1,2 @@
-io.write("ok") // expect: ok
+IO.write("ok") // expect: ok
 /* comment */

test/comments/line_at_eof.wren

@@ -1,2 +1,2 @@
-io.write("ok") // expect: ok
+IO.write("ok") // expect: ok
 // comment

test/comments/unicode.wren

@@ -8,4 +8,4 @@
 
 // TODO: What about combining characters?
 
-io.write("ok") // expect: ok
+IO.write("ok") // expect: ok