Use Cargo workspace inheritance to reduce Cargo.toml duplication

[edmorley]

Rust 1.64's Cargo ships with a new workspace inheritance feature:
https://blog.rust-lang.org/2022/09/22/Rust-1.64.0.html#cargo-improvements-workspace-inheritance-and-multi-target-builds

Amongst other things this allows for specifying the libcnb version once in the repo root Cargo.toml and then having that be used by all crates, which (along with dependency inheritance) would simplify the release process (and also make the check-versions CI check redundant).

It's not clear whether we'd need to bump the MSRV to 1.64, or whether newer Cargo is only needed when working on this repository itself (it's possible the published crates might use the resolved value and so work with older Cargo). Running cargo package locally and inspecting the resultant archives should be able to answer that.

[Malax]

I think we still want the check-versions check since nothing prevents you from not using the workspace version in a package.

[edmorley]

True, though I wonder if it's worth paying the cost (in terms of waiting for checks and having that check clutter up the CI status list) for that given adding new crates is pretty rare? We also don't have a check to ensure that the other workspace attributes are consistent (eg rust-version) for example.

It feels to me like the new workspace feature adds just enough guard rails here that we no longer need to worry about this being an issue moving forwards?