rust: remove newly redundant `use` statements with the 2021 edition prelude https://doc.rust-lang.org/edition-guide/rust-2021/prelude.html

rust: move all crates in the main workspace to edition 2021 We've changed our minimum Rust version to 1.61.0 in the previous patch, and edition 2021 predates that version.

rust: upgrade supported Rust toolchain version A few months ago¹, a decision was made to move the Rust toolchain target to whatever Debian Testing was tracking. I didn't have the bandwidth to act on it until now. This is starting to be even more problematic than before, now that edition 2021 is out. The CI has been updated to track the current Debian testing version, 1.61.0. [1] https://lists.mercurial-scm.org/pipermail/mercurial-packaging/2022-April/000338.html

branching: merge stable into default

tests: fix test-sparse-revlog This one is not covered by the CIbecause I requires an expensive artifact to be cached. So it goes out of think on regular basis (we should fix that…) The test ouput was affected by e706bb41fdb3 as we filtering now happens sooner, removing for the output.

delta-find: adjust the moment when we mark something as "tested" In a coming change, not all elements of `group` might get tested. So we need to have more control about when a revision is actually added to the `tested` set. So we move to a more verbose (and more fragile) version.

delta-find: rename a variable for clarity the index in the delta-chain is also the snapshot depth. So we rename the variable for clarity.

delta-find: small documentation update This is not a 1-1 mapping, but a 1-n mapping. Lets make the associated comment clearer.

delta-find: move pre-filtering with other pre-filtering logic This is more consistent and will help use to be in a clean state before dealing with the "too large group" issue. As a side effect, the debug output now skip some useless cases, making it more useful.

delta-find: expand a function definition and call before extendin it This make the next changeset more compact.