PFA implementation

[dannys4]

I think the major missing algorithm in this package is the prime-factorization algorithm for mixed-radix FFTs where the factors are co-prime. I'm not entirely sure, but I think this is why the composite benchmark is substantially worse than the rest relative to FFTW for small-size ffts.

This one has some serious considerations for a few reasons:

• which radices to choose in a many-composite algorithm?
• How to effectively incorporate this into the callgraph?
• Is this actually any better? I have no idea

For completeness, I should note that I've previously implemented Rader's algorithm at some point (I'm not sure if in C++ or julia...) and it did not work well at all unless your prime is length $2^k - 1$, which I'll leave until someone actually struggles with the use-case.