Consider removing Cargo.lock

[arlyon]

For libraries, it is common to not include the lockfile

https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html#cargotoml-vs-cargolock

https://doc.rust-lang.org/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries

Perhaps we should consider following the convention.