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.

rust-discovery: encapsulated conversions to vec for instance methods This new `pyiter_to_vec` is pretty trivial, and only mildly reduces code duplication. The main advantage is that it encapsulates access to the `index` attribute, which will be changed when we replace the C index by the Rust index, given as `PySharedRef`.

rust-discovery: moving most of hg-cpython methods to regular code blocks The chosen methods are those with conversion of an incoming Python iterable, as they will be changed the most when we will remove the C index, and `takefullsample` for consistency with `takequicksample`.

rust-index: using `hg::index::Index` in `hg-cpython::dagops` Hooking `headrevs` to the Rust index is straightforward as long as we go the `PySharedRef` way. Direct attempts of obtaining a reference to the inner `hg::index::Index` fail for lifetime reasons: the reference is bound to the GIL, yet the `as_set` local variable is considered to be static (the borrow checker clearly does not realize or care that this set only stores `Revision` values). In `rank()`, the chosen solution is the simplest as far as `hg-cpython` is concerned, but it has the defect of removing an implementation that would be easily adaptable if the core index did implement `RankedGraph` (returning the same error as long as only `REVLOGV1` is supported), but that would introduce a direct dependency of `hg-core` on the ``vcsgraph` crate.

rust-index: using `hg::index::Index` in MissingAncestors With this, the whole `hg-cpython::ancestors` module can now work without the C index.