improve + and == operator

- operator+
    adapt operator+ for multi null character.
    {lhs}'s terminal null characters and {rhs}'s
    terminal null characters are put in the end of
    concatted string.
        "hoge\0" + "huga\0\0" -> "hogehuga\0\0\0"

- operator==
    ignore the number of terminal null characters and
    characters behind them.
        "hoge\0\0\0" == "hoge\0huga\0" -> true
get terminal width using TermInfo