[#12EAA76EB6E2BFA59C1F282C8C941D699D6B9E18]
Return the decimal value of a hex character
signed char hexdig_value(char ch) noexcept;
This function returns the decimal value of a hexadecimal character, or -1 if the argument is not a valid hexadecimal digit.
HEXDIG = DIGIT
/ "A" / "B" / "C" / "D" / "E" / "F"
/ "a" / "b" / "c" / "d" / "e" / "f"