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

[David Sparks]

I notice that, as of merge 270ec8c30c48bb01bf7ff41f25e6fda48790b243, FLINT has given up on GMP's mpz_gcd() and rolled its own.  There's no obvious reason GMP can't adopt the same techniques.

(If you want to study the implementation, FLINT just massively overhauled its documentation with AI assistance and would appreciate anyone pointing out flaws in the new docs.)