Pre-RFC: Crate relationships (not namespacing!)

[newpavlov]

A similar idea:

Idea: introduce `project` field to Cargo.toml to make micro-crate designs less scary

In this discussion I've ended up with the following idea. I think it's worth to flesh out it a bit more and have more eyes on it. Motivation Recently the problem of "dependencies explosion" have been raised many times. A natural reaction is to push for limiting or even reversing split of projects into smaller crates (e.g. tokio). While small-crate approach indeed has its issues (e.g. in some cases it can increase maintenance cost a bit or make reviews harder, makes life of Linux package mainta…