[phobos] FFT

[Walter Bright]

Walter Bright wrote:
>
> The most important thing for a Phobos implementation of FFT (or any 
> other module) is getting the API right. Having that right means we can 
> always swap it out for a better implementation without breaking user 
> code.
>

For FFT this would mean looking at the best FFT implementations out 
there to see what their API is. Phobos' should support a full feature 
set, even if for now the implementation will throw exceptions for things 
it doesn't support yet.