add Cargo.lock

As mentioned in #8 having a Cargo.lock can be helpful to bisect
potential issues caused by interactions with external dependencies.

Having a Cargo.lock in the repo allows to achieve a more reproducible
state at a certain point time.

Link: https://doc.rust-lang.org/cargo/faq.html#why-have-cargolock-in-version-control
Suggested-by: @zyklotomic
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>