asciiDigitValue
Determines the numeric value of a character as a decimal digit. If the character is not a decimal digit according to func@GLib.ascii_isdigit, -1 is returned.
Differs from func@GLib.unichar_digit_value because it takes a char, so there's no worry about sign extension if characters are signed.
Return
the numerical value of @c if it is a decimal digit, -1 otherwise
Parameters
c
an ASCII character