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

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.

https://github.com/rust-lang/cargo/issues/9406

Start with the workspace-level `.config/cargo.toml` and review Rust Cargo issue #9406 to confirm whether per-package targets are available in stable Rust. Inspect the workspace packages and current target configuration; the work is done when each package uses its intended target configuration without relying on the workspace-wide setting.

Written by the indexing model from the issue text.