Teach Generated about Cargo lock files

[bb010g]

This PR teaches linguist.generated.Generated about Cargo.lock files.

Resolves #3988.

Checklist:

• I am associating a language (TOML) with a new filename (Cargo.lock) .
  • The new filename is used in hundreds of repositories on GitHub.com
    • Search results for each filename:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=filename%3ACargo.lock+Rust+NOT+nothack
  • I have included a real-world usage sample for all filenames added in this PR:
    • Sample source(s):
      • Aaronepower/tokei/Cargo.lock (licensed under MIT/Apache-2.0)
      • BurntSushi/ripgrep/Cargo.lock (licensed under MIT/Unlicense)
      • BurntSushi/xsv/Cargo.lock (licensed under MIT/Unlicense)
      • Keats/gutenberg/Cargo.lock (licensed under MIT)
      • casey/just/Cargo.lock (licensed under CC0-1.0)
      • google/xi-editor/rust/Cargo.lock (licensed under Apache-2.0)
      • habitat-sh/habitat/Cargo.lock (licensed under Apache-2.0)
      • paritytech/parity/Cargo.lock (licensed under GPL-3.0)
      • pingcap/tikv/Cargo.lock (licensed under Apache-2.0)
      • redox-os/ion/Cargo.lock (licensed under MIT)
      • redox-os/kernel/Cargo.lock (licensed under MIT)
      • rust-lang-nursery/mdBook/Cargo.lock (licensed under MPL-2.0)
      • servo/servo/Cargo.lock (licensed under MPL-2.0)
      • way-cooler/way-cooler/Cargo.lock (licensed under MIT)
  • I have included a change to the heuristics to distinguish my language from others using the same filename.
• I am adding new or changing current functionality (modifying generated lists)
  • I have added or updated the tests for the new or changed functionality.

[pchaigno]

Could you replace this sample with one of the "real" samples you linked to in your post?

[bb010g]

Sure. That one was generated from the hello_cargo example. Sorry, I misunderstood what the PR template meant with the nesting of the sample item. I thought it said that you had to have a sample, and the sources list was more in line with the search verification item.

[lildude]

LGTM. Thanks for the PR and welcome to Linguist.