A Fast Fourier Transform implementation in Standard ML.

Intended to be reasonably fast rather than beautiful -- it's largely
imperative using lots of mutable state.

Includes complex-complex and real-complex FFTs. Supports power-of-two
sizes only. No dependencies apart from the Basis library.

The complex-complex FFT is modelled on Nayuki's multi-language version
The real-complex wrapper is modelled on KissFFT by Mark Borgerding.

Chris Cannam,
Free BSD-style licence, see COPYING for details.