Extending the mpn interface

[Zimmermann Paul]

       Dear Niels,

> But I miss some features in the public interface to do this [...]

I'm glad you hit this issue. I hit it for GMP-ECM and MPFR several years ago.
I reported missing functions in the public interface, and some of them were
added during the years. You have to be patient.

I would add the mpn_redc_{1,2,n} functions: it would be nice to have their
interface fixed once for all.

Paul