Avoid initializer-string warning for the digits array

Avoid initializer-string warning for the digits array

Some newer compilers emit this warning:

```
warning: initializer-string for character array is too long, array size is 36
     but initializer has size 37 (including the null terminating character);
     did you mean to use the 'nonstring' attribute? [-Wunterminated-string-initialization]
 12146 | static char const digits[36] = "0123456789abcdefghijklmnopqrstuvwxyz";
       |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

To avoid the warning initialize the array as individual characters