Hide the `GitOid::new_invalid` constructor.

[alilleybrinker]

When introducing the FFI we added a new constructor for GitOid which allows the construction of an invalid (though still safe to use!) GitOid called new_invalid. Right now the documentation for it says to never call it directly, but on reconsideration it's probably better to hide it from documentation entirely to reduce the likelihood that anyone calls it at all.

We might also consider ways to make it so we don't need to export the symbol, which I believe should be possible. Right now it's pub because of Rust's privacy rules, because the fields of GitOid are private and the constructor needs to access them. One solution would be to put the ffi module (or at least some of it) below the gitoid module, though we may want to do that with module redirects so it doesn't need to be awkwardly placed in the file hierarchy.

[alilleybrinker]

Thinking more about this, we can actually do better. If the FFI module is that to be a child of the gitoid module, then it can access parent scopes. GitOid::new_invalid then wouldn't have to be pub for the FFI-exposed function to access it, and so it could be invisible to Rust code but callable from the FFI.