Return the decimal digit value of a decimal digit character. Such
characters have the general category
Nd (decimal digit
numbers) and a
NumericType_ of
NTDecimal.
No digit values are returned for any Han characters, because Han
number characters are often used with a special Chinese-style number
format (with characters for powers of 10 in between) instead of in
decimal-positional notation. Unicode 4 explicitly assigns Han number
characters a
NumericType_ of
NTNumeric instead of
NTDecimal.