the iterator returned by `utf8.codes` needs some arguments (when called outside a `for` loop)

```
$ cat codes.lua
local s = "foo"
local iter = utf8.codes(s)
print(iter(s))
print(iter(s, 1))
$ ./tl run codes.lua
1	102
2	111
$ ./tl check codes.lua
========================================
2 errors:
codes.lua:3:11: wrong number of arguments (given 1, expects 0)
codes.lua:4:11: wrong number of arguments (given 2, expects 0)
```