fft interface

[Zimmermann Paul]

       Hi GMP developers,

several applications using GMP (in particular GMP-ECM) use the internal fft
functions (mainly mpn_mul_fft and mpn_fft_best_k) for multiplication modulo
2^N+1. This saves a factor of two with respect to mpn_mul_n if for example you
know the lower part of a N-bit product, and you want to compute the upper part.

However those functions are still internal. Is it planned in the future to
make them (or similar functions) part of the public interface?

Best regards,
Paul Zimmermann