Aspect to generate Cargo.toml files for tooling support

Almost all tooling in the rust ecosystem depends on at least `Cargo.toml` files and `cargo metadata`. It would be good to generate the manifests.

https://github.com/mfarrugi/rules_rust/tree/marco-generate-cargo-toml/aspect is a WIP that uses an aspect to generate the information, and then a shell script to move it to the right places. I'm not confident that is the correct way to work with aspects, if anyone has thoughts I would be glad to hear them.

A little bit of the tangent is the aspect uses some of the utilities in toolchain.bzl, it would be good to separate those into another file, are there any qualms with that? And then there is bike-shedding on where the aspect/shell-script should go directory-structure-wise.

Edit:
Updated wip to https://github.com/mfarrugi/rules_rust/blob/marco-generate-cargo-toml/rust/aspect/generate_cargo_workspace.sh