y-crdt - Rust port of Yjs

[https://github.com/y-crdt/y-crdt/]

The yffi crate and its dependency yrs crate are used by LO code.

In order to download all the 144 (currently) dependencies with cargo, a bit
of scaffolding is required: not only the Cargo.lock file that actually lists
all the dependencies, but also all the Cargo.toml files from the tarball,
and empty dummy src/lib.rs files are all required so that cargo fetch stops
complaining and starts downloading.