Use cxx for interop with Faiss

[ava57r]

Hello.

Could We think about to use https://crates.io/crates/cxx for interop?

[Enet4]

This has been brought up in #22. I feel that this would constitute a pretty heavy change, but if someone can manage to make interoperability via cxx work smoothly, then I suppose it can be considered. It is true that trying to keep the C API up to date has been a continuous burden, so I understand the interest in moving away from it.