rust: add explicit resolver field to top-level cargo manifest file
Since Rust 1.51.0, Cargo has support for a better dependency resolver.
Workspace packages (like ours) need to explicitely set the field, so let's.
https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html
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https://www.mercurial-scm.org/wiki/ContributingChanges
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https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
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273ce12ad8f1` will report some style violations on a very old commit).