RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v8]

[Per Minborg]

On Tue, 25 Mar 2025 15:52:07 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> Implement JEP 502.
>> 
>> The PR passes tier1-tier3 tests.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revamp toString() methods

I have rewritten all the `toString()` methods. A `StableList::toString` now produces something much more similar to a regular `List::toString`. The only difference is that the `StableList::toString` shows ".unset" for the elements that are not yet evaluated. In other words, `StableList::toString` no longer evaluates all the elements, but rather does a "high impedance" scan over them and if evaluated, invokes `toString` on the element, otherwise just shows ".unset" for that element.

The same goes for `StableMap` and all the stable functions (which now share the same code path as the stable collections). 

`StableValue` itself does not add extra square brackets around its content.

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PR Comment: https://git.openjdk.org/jdk/pull/23972#issuecomment-2751829347