Difference operator sometimes doesn't work with utf-8

Here's a simple example:
```
$ cat test.c
/*!re2c
   test = [\x20-\x7e] \ [()"; ];
*/
$ .build/re2c test.c -8
re2c: error: line 2, column 32: can only difference char sets
```

Interestingly, it works if I remove characters from the second set:
```
   test = [\x20-\x7e] \ [()];
```