When a Map has a non-indexable key, use a Map (plus refactoring and version bumps)

[aSemy]

Hey, so there are lots of changes in this PR. I recognise there's quite a lot, and so if you'd like I can break them down into smaller PRs so they can be reviewed step-by-step. But first I thought I'd send over the end result rather than flooding you with lots of small PRs!

In summary:

• upgrade versions (Gradle, Kotlin, Spek, Junit)
• add the qoomon.git-versioning plugin (the project's version will be determined by the git tag of the current commit)
• Migrate Gradle to .kts
• apply some project config files (git config, gitignore, editor config)
• some refactoring
  • split up test classes
  • standardise code style
  • remove un-used code
• added more test cases
• added Jacoco code coverage
• if a Map<> has a non-indexable key, use an ES6 Map (Resolves #16)