Updated EA implementing Value Classes and Objects

[Dan Smith]

We've published an updated EA that implements the Value Classes and Objects JEP. See the release notes for details:

https://openjdk.org/projects/valhalla/early-access

The core implementation is now aligned with all of the design changes we've worked on over the last couple of years, including tweaks to the modifiers and flags being used, shifting from Q types to standard nullable reference types, and adopting standard <init>-based construction. This is a significant milestone!

Please note the disclaimer about performance: not all performance optimizations are active in this release, and it is not intended to be used as a reference point for benchmarking.