Saves 0.6% for 64 kb/s and 1.8% for 128 kb/s when decoding on arm7tdmi.

This should prevent extreme dynalloc behaviour in cases where some
bands are heavily attenuated.

Also minor C89 fix for the previous commit

We only keep when concealing less than 10ms with SILK.

Since we're doing two rotations, we can invert the sign on both.
Also adding a few comments for optimizing the FFT.

Also simplifying the code

Undoes commits f7547a4e and 72513f3c

This also happens to increase the accuracy since it appears that the new
ordering is optimal (at least for 20 ms frames), whereas the previous ordering
was pessimal.

IMDCT now uses the forward FFT.

This makes us lose a bit of precision in the first steps, but our gain is more
precise because it's only rounded once. Overall, SNR is slightly improved.