Add SwiftPM support, GitHub Actions CI.

[BrentMifsud]

• Added in support for swift package manager.
• Added Swiftlint and swiftformat config
  • to install SwiftLint: brew install swiftlint, run with swiftlint
  • to install swiftformat: brew install swiftformat, run with swiftformat . --swiftversion=5.2
• Added GitHub Actions to build and lint the project.
• fixed a rare index out of bounds crash.

[BrentMifsud]

The Github action I added is very simple.

As there are no pre-existing test cases, I just run a build, and SwiftLint.

If the build is successful and there are no serious code quality complaints from SwiftLint, the GitHub action will succeed.

For the SPM support, best practice would be to cut a release. On the fork I created, I added a 1.2.6 tag. I would suggest doing the same here once this branch is merged. Otherwise users will have to use a branch or a commit SHA with SPM.