JEP 401 revisions: Null-Restricted Value Object Storage

[Dan Smith]

I've made some revisions to JEP 401 to align with our latest design ideas for expressing flattenability in the language and in class files.

https://openjdk.org/jeps/401

At one point I was considering introducing nullness features in a separate JEP, but the consensus seems to be that we're better off delivering a smaller version of nullness features first, only applicable to value classes. So I've revised the JEP title to be "Null-Restricted Value Object Storage" and eliminated the dependency on a separate nullness JEP.

(Don't forget that the core Value Objects concepts related to identity have been lifted out into their own JEP, https://openjdk.org/jeps/8277163, which is a prerequisite to JEP 401.)