8.5 Digit value
Digit characters are like decimal digit characters, possibly in special forms,
like as superscript, subscript, or circled. The following function converts a
digit character to its numerical value.
— Function: int uc_digit_value
Returns the digit value of a Unicode character.
The return value is an integer in the range 0..9, or -1 for characters that
do not represent a digit.