This can almost certainly be improved further, as less than half of the indices
really need their reversed bit-pattern calculated and elements swapped (any
symetrical bit pattern would just swap with itself, and indices whose reversed
bit-pattern has already been traversed is already swapped).
It may also prove beneficial to provide the base-2 log of the fft buffer size
(number of bits to represent the indices), as that could help make the reversal
more efficient with a known bit/loop count.
Chris Robinson