Convert all fns to block arg syntax.

test/while/closure_in_body.wren

@@ -3,7 +3,7 @@ var list = []
 var i = 1
 while (i < 4) {
   var j = i + 1
-  list.add(fn IO.print(j))
+  list.add(new Fn { IO.print(j) })
   i = i + 1
 }