Adds cargo lockfile

This allows us to ensure that builds is reproductible, since every person will
build the exact same dependency version.

Adding the cargo lockfile for binaries is considered as a good practice,
 according to the cargo book [1].

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

Author: Sasha