Major speed improvement to 1D FFT (MATH-732)

Hi,
Kurt has recently proposed a patch for 1D-FFT which is much faster
than our current impl, while still passing the tests (of course).
Am I likely to hurt anyone's feelings if I replace the old impl by the new
impl?

Also, I think that one of the reasons why our previous implementation
was so slow is because it used internally Complex objects. Since these
objects are immutable, we end up creating *a lot* of small objects.
Apparently, the GC and JIT are not so good at that ;) (not being an
expert, I'm not sure that the culprit is indeed the JIT or the GC...
but surely someone is to be blamed).

I remember we recently had this conversation on the ML: one of Gilles
or Luc argued that for very low-level algorithms, it didn't really
matter if the parameters were passed as "very crude" objects, since it
was most likely that the user would have to write an adapter to its
own data format. So I would suggest we give up Complex[] altogether in
the interface of FFT, and replace it with double[] arrays, with the
following layout :
data[2 * i] = real part
data[2 * i + 1] = imaginary part.

What do you think?
Sébastien