Expose boxed inline value classes in JVM

This is an issue to discuss **exposing boxed inline value classes in JVM**. The current full text of the proposal can be found [here](https://github.com/Kotlin/KEEP/blob/jvm-expose-boxed/proposals/jvm-expose-boxed.md).

We propose modifications to how value classes are exposed in JVM, with the goal of easier consumption from Java. This includes exposing the constructor publicly and giving the ability to expose boxed variants of operations.

Read the linked proposal at proposals/jvm-expose-boxed.md first, then review the discussion on this issue about exposing boxed Kotlin value classes to Java. The work is not ready to start from the issue alone; done would require an agreed proposal and an implementation scope for the public constructor and boxed operations.

Written by the indexing model from the issue text.