Update to Rust 1.70

The last couple of releases were not very interesting for for solving coding challenges, so I didn’t bother updating the test runner.

1.70 has OnceCell and OnceLock, which could be useful. I’m thinking mostly of robot-name, where a lot of people use lazy_static or the crate once_cell for some global initialization.

@ErikSchierboom Update to Rust 1.70 by remlse · Pull Request #73 · exercism/rust-test-runner · GitHub