Dependabot raised a soundness issue with Rust lexical-core dependency

[patchwork01]

See Dependabot alert:
https://github.com/gchq/sleeper/security/dependabot/24

This seems to indicate there's an issue that could be picked up by cargo audit, which we're using in GitHub Actions to check for security issues in our Rust code. See also:

https://github.com/rustsec/rustsec/blob/main/cargo-audit/README.md
https://rustsec.org/advisories/RUSTSEC-2023-0055

The cargo audit documentation says it uses the RustSec database, and the RustSec database seems to agree with Dependabot. We can see if we need to configure cargo audit further to fail on this sort of problem.