cargo-dist (and cargo-release)

[tshepang]

Ease release process by introducing cargo-dist and cargo-release to release flow.
In addition, bindgen-cli is also published as a binary.

[tshepang]

Docs are now fixed, and this should be ready for review.

To explain the release workflow (assuming this PR gets accepted)...

• install cargo-release
• when doing a feature release, run cargo release minor, which does a dry-run of what would happen
• run cargo release minor --execute, which will
  • publish bindgen and bindgen-cli to crates.io
  • tag and push the commit
    • this will trigger the release workflow, which will create installers and tarballs for bindgen-cli