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.