Draft: Goal for gf2x-2.0: rewrite in such a way that it uses `mp_limb_t`'s, and integrates better with gmp

There's still much to do, but let's keep track of a few things.

• In passing, we want to factor out the testing infrastructure, since so much of it is common with cado-nfs
• We have some warnings on OSX
• Make sure we have at least the very basic plumbing that the new mp2x type calls for.
• Write tests for those.
• Ideally, there should be some doc, right?

Beyond that, there's the question of how far we want to go. Do we want a division function, for instance?