When available in stable Rust, switch to using per-package targets

[weegeekps]

Today, for the entire cargo workspace we're defining the target triple in the workspace level .config/cargo.toml file. This is okay for now, but as this monorepo matures we likely want to use per-package targets for each package. Unfortunately, per-package targets are still being worked on by the Rust team, so we'll have to wait for them to finish their work.

rust-lang/cargo#9406