Add Cargo.lock ?

[tomtau]

@liamsi #68 (review):

Shouldn't we rather use the latest release (2.6.2) instead (and regenerate the rust code using that release)?

Regarding the lock file, https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html states 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. If you’re building an end product, which are executable like command-line tool or an application, or a system library with crate-type of staticlib or cdylib, check Cargo.lock into git.

rust-abci is more of a library and not an end-product. At the same time, I think adding the cargo lock file would increase reproducibility. Which seems important for rust-abci IMHO. Let's discuss this in a separate issue.