Restore Cargo.lock

[QIU1995NONAME]

• I agree to follow the project's code of conduct.
• I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

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[michaelkirk]

I'm personally a fan of including the Cargo.lock, mostly because it makes it easier to build historic commits when you need to do ancient code archeology. It does also remove unexpected drama from PR's (like #211).

The main downside I see, is by pinning old versions in a lockfile, CI might hide build problems that real users today are having.

On balance, I still think we should merge, but would want others to weigh in.

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The immediate failure was:

error: package syn v2.0.109 cannot be built because it requires rustc 1.68 or newer, while the currently active rustc version is 1.63.0

So instead of, or in addition to, committing the Cargo.lock, we could raise our MSRV.

[michaelkirk]

Maybe also related/solved by https://github.com/georust/rstar/pull/204/files ?

[adamreichold]

AFAIK, checking in Cargo.lock is the new recommendation, c.f. https://doc.rust-lang.org/cargo/faq.html#why-have-cargolock-in-version-control. We could have a CI job that builds after deleting that file if we fear breakage from newer versions.