rust-index: core impl for find_gca_candidates and find_deepest This still follows closely the C original and not able to treat more than 63 input revisions (bitset backed by `u64` and one bit reserved for poisoning).

rust-index: add support for `reachableroots2` Exposition in `hg-cpython` done in regular impl block, again for rustfmt support etc.

hg-cpython: rev_pyiter_collect_or_else It will be useful to give callers the control on the generated errors

rust-index: add support for `computephasesmapsets` Exposition in `hg-cpython` done in the regular `impl` block to enjoy rustfmt and clearer compilartion errors.

rust-index: slicechunktodensity returns Rust result Ready for removal of the scaffolding. This time, we allow ourselves a minor optimization: we avoid allocating for each chunk. Instead, we reuse the same vector, and perform at most one allocation per chunk. The `PyList` constructor will copy the buffer anyway.

rust-index: add support for `_slicechunktodensity`

rust-index: headrevsfiltered() returning Rust result

rust-index: add support for `headrevsfiltered` The implementation is merged with that of `headrevs` also to make sure that caches are up to date.

rust-index: implement headrevs

rust-index: variant of assert_py_eq with normalizer expression The example given in doc-comment is the main use case: some methods may require ordering insensitive comparison. This is about to be used for `reachableroots2`