Add Cargo.lock into .gitignore

[Yuuki Rika]

From this documentation https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html, it's stated that

If you’re building a non-end product, such as a rust library that other rust packages will depend on, put Cargo.lock in your .gitignore.

The GitHub gitignore convention uses it: https://github.com/github/gitignore/blob/main/Rust.gitignore

...
# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
Cargo.lock
...

[Philipp Oppermann]

Thanks for raising this issue! I'm aware of the official recommendations, but I'm still in favor of keeping the Cargo.lock checked in for two reasons:

1. There are plans to change the official recommendations to always check in the Cargo.lock. See:

• rust-lang/cargo#8728
• rust-lang/cargo#12275

2. This project consists of multiple crates that are combined together. Most of them are built as binaries. So even with the current recommendation, we should keep the Cargo.lock checked in.

[Yuuki Rika]

All right, I'll hold off the pull request. Thanks for the review!