Scoped Values (Fourth Preview)

[Andrew Haley]

One change:

The fluent API that is described in the Javadoc is now the only API supported.
The previous "shorthand" form is gone, simplifying the API. The only way now
to invoke Java code with a bound ScopedValue is via the
ScopedValue.Carrier.call() and ScopedValue.Carrier.bind() methods. This results
in a small API surface, which I rather like.

The draft JEP is at
https://bugs.openjdk.org/browse/JDK-8338456

I pushed the source changes to the fibers branch.

Alan (Bateman), will you please put the Javadoc somewhere and send me a link,
so I can link it to the PR? Thanks.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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