hg-core: remove unneeded trait now that we support Rust 1.52+

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.