contains_char("", 'X') ==> false.
contains_char("abc", 'X') ==> false.
contains_char("abc", 'a') ==> true.
contains_char("abc", 'b') ==> true.
contains_char("abc", 'c') ==> true.
contains_char("aßξ啕𐀀.", 'ß') ==> true.
contains_char("aßξ啕𐀀.", 'ξ') ==> true.
contains_char("aßξ啕𐀀.", '啕') ==> true.
contains_char("aßξ啕𐀀.", '.') ==> true.
contains_char("aßξ啕𐀀.", '☿') ==> false.
contains_char("abc�123", 'c') ==> true.
contains_char("abc�123", '1') ==> true.
contains_char("abc�123", '2') ==> true.
contains_char("abc�123", '3') ==> true.
contains_char("abc�123", '�') ==> false.