Implement FFT or another faster multiplication algorithm than Toom-4

[peteroupc]

To do this, a step-by-step description of FFT multiplication, together with examples, would be helpful for testing purposes. However, most of the descriptions of FFT integer multiplication I've seen involve complex numbers (including pi and the exponential function, when I know it could be done with only integer arithmetic), or they include no examples allowing someone to test their FFT multiplication implementation. Because of this, my current attempts to implement FFT integer multiplication have failed.