--- Iconv#iconv(str, [ start = 0, [ length = -1 ] ])
    ʸѴ򳫻ϤѴʸ֤ޤ

    str ʸξ硢str[start, length] Ѵ
    Ѵʸ֤ޤ

    str  nil ξ硢Ѵ򤽤νեȾ֤ˤ
    ϥХåեեȾ֤᤹ΥХ󤫤ʤʸ֤ޤ

    ¾ξ㳰򵯤ޤ

    str Ѵʸޤ nil Ǥ

    start  str ΤѴ򳫻Ϥ륪եåȤꤷޤ

    length  str ΤѴĹǡ
    nil  -1 ΤȤϡstart ʹ̣ޤ

    Υ᥽åɤϵ㳰
    Iconv::IllegalSequence
    Iconv::InvalidCharacter
     Iconv::OutOfRange
    Ǥ

