Submitted JEP: Extent-Local Variables

[mark.reinhold at oracle.com]

https://openjdk.org/jeps/8263012

Andrew & Andrew,

First, thank you!  This is great work, and along with the rest of Loom
it’s going to change the world.

I’ve done a light editing pass to tighten the prose a bit, fix some
references, and make the terminology and notation consistent with
related JEPs.

I also changed the term “broadcast” to “transmission,” since the former
suggests that any thread can receive the information when really only
descendants of the thread that binds an extent-local variable can read
it.

As written, the JEP implies that `ExtentLocal` has a `run(...)` method,
but of course it doesn’t -- that’s declared in `ExtentLocal.Carrier`,
which you don’t mention anywhere in the JEP.  I’m guessing that you
chose not to mention the latter class so as to avoid making the JEP even
longer.  I’ve thus tried in the text to weaken the implication that
`ExtentLocal` declares `run(...)`.  Since the carrier class enables
chained `where(...)` invocations, which seem very useful, I do recommend
that you consider explaining it in a short subsection just before the
migration discussion.  (It’d be fine to continue to suppress mention of
it earlier on.)

Diffs: https://cr.openjdk.java.net/~mr/jeps/jep-extent-local-variables-i1-01a_diff.html

Please review my edits, make any necessary corrections and adjustments,
and then I’ll move this JEP to Candidate.

- Mark