Implement `mpn_gcdinv` with precomputed powers of two modular inverses

[albinahlback]

Should allow for fast iterations via the binary Euclidean algorithm. Would probably work very well with the mpn_mod module.

[fredrik-johansson]

What algorithm precisely? The binary Euclidean algorithm expressly avoids divisions, and in the ordinary Euclidean algorithm the divisor changes each iteration so precomputations don't seem useful.

[albinahlback]

Modified version of Algorithm 1 of Thomas Pornin's paper