[Improvement]: bump Shuttle's pinned rust version to 1.70

[oddgrd]

Describe the improvement

We should bump the pinned version of rustc to 1.70 (in the Makefile and CI). With 1.70 (and possibly 1.69) also came some new language features, we should see if we can benefit from implementing some of these. For example in 1.70 OnceCell was added to the standard library, and a new method on options/results: is_some_and/is_ok_and. If there are any major changes we should implement, it should be a separate PR.

PS. in 1.70 sparse registries is enabled by default, so we can remove the CARGO_REGISTRIES_CRATES_IO_PROTOCOL: "sparse" env from our CI.

Release notes:

• https://blog.rust-lang.org/2023/06/01/Rust-1.70.0.html
• https://blog.rust-lang.org/2023/04/20/Rust-1.69.0.html

Duplicate declaration

• I have searched the issues and this improvement has not been requested before.

[iamwacko]

LazyLock is still experimental, and that is what we use OnceCell for, so the OnceCell inclusion won't affect much.