TODO before FFTW-$2\pi$:

* MPI version

* DCT/DST codelets?  which kinds?

* investigate the addition-chain trig computation

* I can't believe that there isn't a closed form for the omega
  array in Rader.

* merge genfft-k7 generator with the main genfft branch.
  genfft-k7 was written by Stefan Kral based on the fftw-2.1 genfft.

* implement rdft/problem2 for even radices other than 2.

* convolution problem type(s)

* Explore the idea of having n < 0 in tensors, possibly to mean
  inverse DFT.

* still too much code duplication in rader

* better estimator: possibly, let "other" cost be coef * n, where
  coef is a per-solver constant determined via some big numerical
  optimization/fit.

* vector radix, multidimensional codelets

* it may be a good idea to unify all those little loops that do
  copying, (X[i], X[n-i]) <- (X[i] + X[n-i], X[i] - X[n-i]),
  and multiplication of vectors by twiddle factors.

* Timed planner: run as ESTIMATE, then IMPATIENT, then PATIENT, then naive?
  PATIENT then IMPATIENT then ESTIMATE would allow the less-patient planners
  to benefit from more-patient sub-plans, on the other hand.

* Pruned FFTs (basically, a vecloop that skips zeros).

* Try FFTPACK-style back-and-forth (Stockham) FFT.  (We tried this a
  few years ago and it was slower, but perhaps matters have changed.)

* Modify generator to produce integer FFTs (etc.) of small sizes.

* dif, difsq simd codelets

* Bluestein's algorithm for prime sizes (and sizes with large prime factors).

* Generate assembly directly for more processors, or maybe fork gcc.  =)

* Find a better DCT-I algorithm.