[All] Java interoperability support

Although Kotlin is [mostly interoperable with existing code](https://kotlinlang.org/docs/java-to-kotlin-interop.html), there are some caveats:

* Calling `suspend` functions is impossible from Java as the [`Continuation` interface appears to expect a `sealed` class that has an `internal` Kotlin class](https://stackoverflow.com/questions/52869672/call-kotlin-suspend-function-in-java-class)