Crux v0.12

MIR updates

This release supports version 8 of mir-json's schema.

• Support simulating Rust code up to version 1.91.
• Counterexample models are now pretty-printed instead of emitted as JSON.
• Align the Rust language edition used by the test suite’s rustc invocation with mir-json (now defaults to Rust 2021), enabling tests that rely on post-2015 language features.
• Add a --mir-json-arg option for passing extra arguments to mir-json.
• Support using async fn and #[coroutine].