rust-index: stop instantiating a C Index The only missing piece was the `cache` to be returned from `revlog.parse_index_v1_mixed`, and it really seems that it is essentially repetition of the input, if `inline` is `True`. Not worth a Rust implementation (C implementation is probably there for historical reasons).

rust-revlog: using the ad-hoc `NodeTree` in scmutil Now that we have an independent `NodeTree` class able to work natively on the pure Rust index, we use it in `mercurial.scmutil`, with automatic invalidation after mutation of the index. This code path is tested by `test-revisions.t` and `test-template-functions.t`

rust-revlog: add invalidation detection to `NodeTree` class This will be useful for callers, such as `scmutil` who reuse a `NodeTree` instance as a cache. They would otherwise get hard errors if any mutation of the index occurred since instantiation. This is something the C index does not provide.

rust-index: add support for `del index[r]` Only the `del index[r:]` syntax was supported, but the comment said otherwise. It's not actually used in core code, but the C index supports it.

rust-revlog: bare minimal NodeTree exposition The independent `NodeTree` instances needs to be associated to an index (for forward-checks of candidates) but do not need to encompass all revisions from that index. This is exactly how it is used in `scmutil.shortesthenodeidprefix` and we restrict the implementation to the bare minimum needed there and to write convincing tests. It would of course be fairly trivial to add more.

rust-index: a property to identify the Rust index as such Will be useful soon in `mercurial.scmutil` and potentially elsewhere

rust-cpython-revlog: renamed NodeTree import as CoreNodeTree We're about to introduce a `NodeTree` Python class (hence also a Rust struct) and it would be a collision with the import

rust-index: stop using C index We still keep its wrapper implementation in `hg-cpython::cindex`, because we might want to recreate ancestors handling objects using it for the case of REVLOGV2. Also, we still instantiate it (from Python code) and store it as attribute, for the likes of `get_cindex` and the caller that relies on it, but that is soon to be removed, too.

rust-index: using `hg::index::Index` in discovery At this point the C index is not used any more: we had to remove `pyindex_to_graph()` to avoid the dead code warning.

rust-python-testing: separated base test classes This will allow, e.g., to change `test-rust-discovery.py` simply by adding the appropriate base class.