Introduce Verus

[nspin]

Verus is a tool for verifying the correctness of code written in Rust.

This PR introduces Verus into the project:

• Build system changes to enable the use of multiple Rust toolchains via the rustEnvironment construct in Nix
• The introduction of the Verus tools into the build system
• A trivial example/test (more to come)

This work depends on a PR to the upstream Verus repository:

https://github.com/verus-lang/verus/pull/1138

For now, we are using this branch of Verus, which includes that patch:

https://github.com/coliasgroup/verus/tree/dev

[lsf37]

Loving the influx of verification tools! :-)