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`.