(Frechet) derivative of the Fourier transform

Stream: PR reviews

[Alex Kontorovich]

In #9663, @Heather Macbeth and I compute the Frechet derivative of the Fourier transform. (With many thanks to @Yury G. Kudryashov for the assist!) It should be possible to iterate this to show that the Fourier transform of a Schwartz function is Schwartz.