Use the newest Multiplatform project structure

[MarkusAmshove]

Kluent currently uses a Gradle structure for realizing multiplatform that is pretty dated.
Since the first days of multiplatform targeting, the way to write multiplatform libraries with Gradle has changed significantly, making the Gradle project a lot more simple.

Migration to the new structure e.g. allows to specify JVM toolchains, which would prevent things like #220 in the future.