Auto-format code with "cargo fmt" and check in CI

[micolous]

This auto-formats the entire repository with cargo fmt (rustfmt) and checks it in CI, in line with the Rust Style Guide's recommendation (previously known as RFC 2436):

Formatting code is a mostly mechanical task which takes both time and mental effort. By using an automatic formatting tool, a programmer is relieved of this task and can concentrate on more important things.

This is using the formatter's default settings, as it's not clear what the existing style standards apply to this project, and that's at least fairly consistent with the rest of the Rust ecosystem.

These PRs are where it looks like a code formatter was run (not necessarily full rustfmt, but even trivial things like stripping trailing whitespace), leading to larger diffs in unrelated code:

• Update to use 2021 edition, and update deps #544
• Add CGDisplayCreateImageForRect function  #547
• Merge upstream changes from Warp's core-foundation-rs fork #517
• implement full NSMenu and NSMenuItem APIs #507
• feat: add link feature #608

By using an auto-formatter, it means that it's much easier to write PRs that merge cleanly without noise, while also laying everything out consistently.

This PR is split into two commits:

• 7b3caa7 adds cargo fmt --check to CI (.github/workflows/rust.yml), which you can see failing here: https://github.com/micolous/core-foundation-rs/actions/runs/5515554024/jobs/10055989579

• 7cee916 auto-formats the code, and was generated with:

  cargo fmt
  git add .
  git commit -m 'Run "cargo fmt" (rustfmt) over core-foundation-rs'

  With that, CI now passes: https://github.com/micolous/core-foundation-rs/actions/runs/5515564815

Because the fix-ups are a huge diff that'll be difficult to review (and would break if there are any other changes merged prior), you might want to cherry-pick the first commit and run cargo fmt yourself, or otherwise swap it around.

[bors-servo]

☔ The latest upstream changes (presumably 127f2be) made this pull request unmergeable. Please resolve the merge conflicts.

[micolous]

Rebased at 127f2be, the CI change is now 163fa33, and I regenerated the auto-formatter commit as 7cc1579.

[micolous]

It looks like a bot needs a poke to push this forward, and I don't have write access to push things, even if they have the auto-merge bit set.