Rexport functions from AbstractFFTs

[ssfrr]

I'm assuming that folks who want to use fft and friends with their CuArrays are supposed to do

using AbstractFFTs
using CuArrays

To bring the FFT API into their namespace. The CUArrays tests actually have using FFTW (which uses @reexport to export the FFT API) so they don't need to pull in AbstractFFTs, but I assume users who aren't doing CPU FFTs wouldn't want that dependency.

AbstractFFTs says in its README:

This package is mainly not intended to be used directly. Instead, developers of packages that implement FFTs (such as FFTW.jl) extend the types/functions defined in AbstractFFTs.

It seems like it would be nice for CuArrays to also re-export these names. This also provides access to fft utility functions like fftshift which are implemented and exported from AbstractFFTs.