Automatically enable parallel compilation in Rust toolchain

`rustc` is now able to do [multi-threaded compilation](https://blog.rust-lang.org/2023/11/09/parallel-rustc.html).  We should add the appropriate configuration to our `${CARGO_HOME}/config.toml` file whenever this feature comes to a stable release. Upstream issue about making this feature stable: https://github.com/rust-lang/rust-project-goals/issues/121