Chapter 2 Programming A Guessing Game

When there are dependencies, the cargo build will automatically fetch the dependent crate in Cargo.toml.
It will create a Cargo.lock to memorize the version of each dependent crate in the first run.
It is practical to include the Cargo.lock file in the source code control system. For consistency and reproducibility, the cargo will use the same version in the following compiling, even there may be a new version uploaded.
Using the cargo update command to update the crate.