[v3] What concept exercises we can provide for Kotlin track

[About concepts](https://exercism.github.io/v3/#/docs/concept-exercises) | [C# example](https://exercism.github.io/v3/#/languages/csharp/exercises/concept/README)

My idea is to make exercism concepts close to the [official language documentation](https://kotlinlang.org/docs/reference/):
- Kotlin reference is quite good;
- it would be easier to track missing / update outdated things;
- reference documentation is kinda built around describing different kind of concepts.

Ideas are more than welcomed! @exercism/kotlin