Fix benchmarks build

[iamnotacake]

One of dev dependencies pulls rayon 1.7.0, and its dependency rayon-core 1.11.0 now requires rust 1.59+

Freeze rayon so it will pull a bit older rayon-core

See failed job for the cause of this PR

Checklist

• Change is covered by automated tests
• Benchmark results are attached (if applicable)
• The coding guidelines are followed
• Public API has proper documentation
• Example projects and code samples are up-to-date (in case of API changes)
• Changelog is updated (in case of notable or breaking changes)

[vixentael]

@ilammy wdyt

[ilammy]

I don't think this fixes the issue. RustThemis CI jobs didn't run here, but I'm thinking they won't pass.

rust-version in Cargo.toml only instructs Cargo to fail compilation earlier instead of when reaching the dependency. It does not make the new version auto-install or anything like that.

The linked job fails because one of the transitive dependencies requires Rust 1.59 now. I see three ways around it:

1. Bump our MSRV to 1.59, don't bother.
2. Pin that rayon/rayon-core dependency to an older version in Cargo.toml of benchmarks. Unpin when we bump MSRV.
3. Kick the benchmarks out of the top-level workspace, test them separately with only stable Rust.

[Lagovas]

so, lets pin version of the dependency? can we do it, @iamnotacake ?