rust-index: optim note for post-scaffolding removal

rust-index: check that the entry bytes are the same in both indexes This is a temporary measure to show that both the Rust and C indexes are kept in sync. Comes with some related documentation precisions. For comparison of error cases, see `index_entry_binary()` in `revlog.c`.

rust-index: return variables systematic naming convention To help knowing at a glance when a method is ready, making us more comofortable when we are close to the final removal of scaffolding, we introduce the systematic variable names `rust_res` and `c_res`. The goal of this series is to always return the formet. We take again the case of `pack_header` as example. Our personal opinion is to usually avoid such poor semantics as `res`, but usually accept it when it close to the actual return, which will be the case in most methods of this series. Also, the name can simply be dropped when we remove the scaffolding. To follow on the example, the body of `pack_header()` should become this in the final version: ``` let index = self.index(py).borrow(); let packed = index.pack_header(args.get_item(py, 0).extract(py)?); Ok(PyBytes::new(py, &packed).into_object()); ``` in these cases it is close to the actual return and will be removed at the end entirely.

rust-index: results comparison helper with details This is a bit simpler to call and has the advantage of systematically log the encountered deviation. To avoid committing dead code, we apply it to the `pack_header` method, that was already returning the Rust result.

rust-index: helper for revision not in index not involving nodemap This is a good match for exceptions raised from the C implementation, when it is not about a nodemap inconsistency.

rust-index: renamed nodemap error function for rev not in index The function name was misleading, as the error wording mentions the nodemap, hence would not be appropriate for missing revisions not related to a nodemap lookup.