Format repository and add test to workflow

[nickfarrow]

fixes #42

This should be merged only after all substantial code change PRs have been merged so we can avoid rebases.

(I'm also testing the github CI using this PR)

[DanGould]

ready for review after rebase?

[DanGould]

cargo +nightly fmt
git diff nickfarrow/format

looks clean to me ACK 6ea4bb4

Doesn't this checkout an extra time? What's stopping you from running the following at the end of the build job as just a step?

- uses: https://github.com/actions-rs/toolchain/tree/v1v1
        with:
          profile: minimal
          toolchain: nightly
          override: true
          components: rustfmt
      - uses: https://github.com/actions-rs/cargo/tree/v1v1
        with:
          command: fmt
          args: --all -- --check