Format: '0x' <hexa> <hexa> where <hexa> ::= 0 | 1 | ... | a | A | b | B | ... | f | F.
Note that it's only for bytes, i.e. values between 0x00 (0) and 0xFF (255) that have the Byte type, everything else should fail.
Specifically, this notation is not for integers of type Int, it's just for us being able to write 0xEF directly instead of relying on 239.toByte.