Preserve case of input digits in error messages.

Save copies of the input number and input digit set before case-folding
them, so that error messages can use the original case of the digits.

Concomitantly, use the original letter case of alphabetic digits when
reporting errors concerning them, even if case-insensitivity is on.
Don’t unnecessarily use postfix `++` and `--`.

Use prefix increment and decrement instead.