Add cargo example for Rust project

[rhysd]

This PR adds example for Cargo, which is an official build tool for Rust toolchain. In addtion to local target build directory, ~/.cargo/registry (packages registry) and ~/.cargo/git (packages index) were necessary to be cached for reusing cached build directory.

I already tried this on my project kiro-editor and cargo test was improved as follows.

• Before (without cache): job
  • Linux: 1m26s (1m32s in total)
  • macOS: 41s (1m4s in total)
• After (with cache): job
  • Linux: 13s (33s in total)
  • macOS: 18s (35s in total)

6.6x faster on Linux.

[rhysd]

This diff occurred since original file did not have newline at the end of file.

[smorimoto]

I prefer to add the hash of the lock file to the key. What do you think?

key: ${{ runner.os }}-cargo-registry-${{ hashFiles(format('{0}{1}', github.workspace, '/Cargo.lock')) }}

or

key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}

[rhysd]

@imbsky

Cargo handles outdated libraries in build directory. For example, when we update some dependency and Cargo.lock is changed, cargo test checks local dependencies and rebuilds packages which were updated. So if I'm correct, we don't need to care and can use cached directory always.

Ah, I misunderstood cache behavior. We need to add hash to key. Thank you for the point. I'll fix it. I think **/Cargo.lock is enough since it changes when dependencies change.

[smorimoto]

Oh, I'm glad if my suggestion helped 😁

[smorimoto]

The example has been moved to examples.md. See #13.

[rhysd]

Thanks. I moved the example to examples.md and rebased this branch onto the latest master.

[smorimoto]

Looks good to me.

[chrispat]

@rhysd can you resolve your merge conflicts and we will get this mrged today.

Thanks.

[rhysd]

@chrispat I did.