FLINT has a new, faster, integer GCD. Adapt for mpz_gcd()?

[Fredrik Johansson]

To clarify: FLINT has a port of GMP's GCD code (all credit to the GMP team)
with only some small tweaks to the actual GCD algorithm.

The speedup (~1.1x for small operands, 2-3x for large ones; slightly more
multithreaded) derives almost entirely from FLINT's faster integer
multiplication (tables of straight-line NxM kernels for schoolbook
multiplication, small-prime FFT above a couple hundred limbs) along with
FFT-space 2x2 matrix ops.

Fredrik