Adding workspace dependencies

[rodoufu]

I've only added the dependencies as workspace dependencies to make it easier to maintain from now on.

I've executed in the serde directory.

$ cargo test --features derive

And in the test_suite:

cargo +nightly test --features unstable

Both worked fine.

As required in the https://github.com/serde-rs/serde/blob/111803ab0768d010c606f2fc0d0add12750d5eef/CONTRIBUTING.md

[dtolnay]

Thanks!