add `#[derive(Eq,Hash)]` to `VecN<T>`

It's handy to be able to use things like `Vec3u` as a hashmap key. As the derives work by adding `impl Hash for VecN { ... }` etc, non `Hash`/`Eq` types (e.g. Vec3f) are unaffected.

Find the definition of VecN and inspect its existing derives. Add the requested Eq and Hash derives so Vec3u can be used as a hashmap key while types such as Vec3f remain unaffected; verify the behavior with the project's existing checks.

Written by the indexing model from the issue text.