fft的快速实现，核心部分无任何耗时的sin,cos运算，所需要的数据初始化时全部已经计算好了。并且提供了fft的定点代码（这是很多fft不具有的），可以方便地移植到嵌入式和DSP环境-fft of rapid implementation, the core portion without any time-consuming sin cos operation, the initialization of the data needed all been calculated. And provide fft fixed-point code (which many the fft do not have), and can be easily ported to the embedded and DSP environment