Add an FFT convolution effect.

A lot of the later commits have been leading up to this, and I finally
got to the point where all the unit tests check out, everything seems
to work (modulo maybe some overflow issues) and we have a model that
matches what people actually expects from convolutions.

Note that this adds a dependency on FFTW3; we could probably have added
our own routines for such small needs, but like with Eigen, calling out to a
library is fine as long as it's of good quality (which FFTW certainly is) and
is widely available.