AVX IFMA Accelerated `split_mul`

I prototyped a portable split_mul implementation premised on using the AVX IFMA instructions to accelerate itself. I believe I optimized it decently, the code is well-documented, the benchmark is easy to run, and the README explains the challenges moving forward. Unfortunately, despite observing a 20-40% improvement when compiled exclusively for the native CPU, I was unable to see any real-world performance benefit when I patched my crypto-bigint dependency to a fork which used this split_mul implementation when reasonable.

https://github.com/kayabaNerve/avx-ifma-mul

I'd love to see further discussion and potential upstreaming (again, as explained in the README). I would have entirely made this an issue if I did not want to provide the implementation + benchmark as an artifact (hence the repository).

Start with the linked avx-ifma-mul repository, reading its README and running its benchmark to understand the proposed split_mul implementation and reported results. Then inspect how crypto-bigint currently implements split_mul and assess whether the prototype can be evaluated for upstreaming, with real-world performance impact established before considering the work done.