Use flat array instead of creating a fft object

[echo66]

The output of the forward FFT is the following: [r1, i1, r2, i2, ..., rN, iN].

The input of the inverse FFT is the following: [r1, i1, r2, i2, ..., rN, iN].

So, instead of creating an intermediary structure, the code should support this kind of arrays.