Cargo-workspace

[max-sixty]

I think this makes the repo consistent with RFC-1525, for multi-crate single-repo projects:

• A Cargo.toml in the root listing the workspace members
• Each crate with their own Cargo.toml without a workspace section
• A single Cargo.lock

Needs a review from someone who has done this before

[nathansobo]

Cool! This seems reasonable. We're going to do some reorganizing but I'll merge this and then update it as we move stuff around.