Project Valhalla value classes support

[zhelenskiy]

This is the discussion related to the Project Valhalla value classes support. The current full text of the proposal can be found here.

It is proposed to compile Kotlin value classes (full and inline value classes, value objects, abstract and sealed value classes) to Project Valhalla value classes (JEP 401) on the JVM under the experimental -Xvalhalla-value-classes flag, which requires JVM target 28 or later and -Xjvm-enable-preview. The proposal also covers interoperability with Java value classes and the value classes of the JDK, and the LoadableDescriptors attribute.