Add character literals to the interpreter and compiler. In factor() / c_factor(), when we see a single quote, read the byte inside and return/emit its integer value. 'a' is 97, '0' is 48. Agreement tests:

    check_both("'a'", 97);
    check_both("'0'", 48);
    check_both("'z' - 'a'", 25);

In the self-hosted compiler, detect single quotes with cur() == 39 (since character literals don't exist yet during bootstrap).