Add faster, sparse polynomial GCD implementation

[mentat-collective]

Add faster, sparse polynomial GCD implementation

[littleredcomputer]

I have Zippel; it's very hard to implement (as GJS himself notices, having inferred a simpler version of the algorithm in Scmutils that avoids doing the work in Z_p). I had this going on a branch, but the benchmarking I did revealed no meaningful speedup. The CAG paper looks more interesting (and postdates my last deep foray into this area), so it might be worth a try! Thank you for that pointer.

[littleredcomputer]

I found this on Google Scholar and it looks very promising:

Speeding up polynomial GCD, a crucial operation in Maple

[sritchie]

Wow, that does look really good!