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.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).