Separate crate nix c bindings

[Radvendii]

At some point we should split nix-c-raw off into its own crate. And also the more rust-y bindings. IIRC those have to be separate crates. Not urgent, just wanted to put it on the "roadmap"

[roberth]

They're technically separate crates but they share a lock and they're in the same repo.
I'd move all nix-* crates to a single new repo when nixops4 is MVP and we know that most bindings development is done. At that point it's better to trade our convenience for the benefits of them being a more visible, independent project.