BITPOS key bit [start] [end]
返回键 key 对应的字符串值中第一个被设置为 1 或 0 的位（Bit）的位置。
Return the position of the first bit set to 1 or 0 in a string.
命令执行时，会将该字符串当成由从左至右所有位（Bit）组成的数组，即第 1 字节（Byte）的最高位（Significant bit）在位置 0 处，而第 2 字节的最高位在位置 8 处，以此类推。
The position is returned thinking at the string as an array of bits from left to right where the first byte most significant bit is at position 0, the second byte most significant big is at position 8 and so forth.
当然，也可以通过指定附加参数 start 及 end 来确定一个区间以用于查找位。
By default all the bytes contained in the string are examined. It is possible to look for bits only in a specified interval passing the additional arguments start and end (it is possible to just pass start, the operation will assume that the end if the last byte of the string. However there are semantical differences as explained later). The range is interpreted as a range of bytes and not a range of bits, so start=0 and end=2 means to look at the first three bytes.
Note that bit positions are returned always as absolute values starting from bit zero even when start and end are used to specify a range.
Like for the GETRANGE command start and end can contain negative values in order to index bytes starting from the end of the string, where -1 is the last byte, -2 is the penultimate, and so forth.
Non-existent keys are treated as empty strings.