fix utf8.codes signature

[fperrad]

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)

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[hishamhm]

Thanks!!