BigInteger:  small optimization to bitwise_not

See https://github.com/phpseclib/phpseclib/pull/431#issuecomment-50237254
Wasn't the idea to use `strlen($this->toBytes())` instead of `strlen($this->toBits()) / 8` and then `strlen($temp)` instead of `strlen($this->toBytes())`? So you would have to remove the `/ 8`.