fft interface

[Zimmermann Paul]

       Hi,

> It definitely makes the wrap-around trick useful for fairly small sizes.

indeed. I've changed the development version of GMP-ECM to use mpn_mulmod_bnm1
instead of mpn_mul_fft for the wrap-around trick.

However we also use mpn_mul_fft when the input number is a Fermat number, and
in that case we can't use mpn_mulmod_bnm1 instead. Is it planned to add
mpn_mulmod_bnp1 to the public interface too?

Paul