Editorial: Ensure Abstract Closures don't re-use outer aliases

[jmdyck]

Resolves issue #3010.

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Usually, I rename the inner alias, but in one case, I rename the outer...

• In AsyncFromSyncIteratorContinuation and Await: change the AC's _value_ to _val_.
• In Promise.prototype.finally: change both ACs' _promise_ to _prom_.
• In GeneratorStart and AsyncGeneratorStart: change the AC's _genContext_ and _generator_ by prepending 'ac'.
• In AsyncBlockStart: change the AC's _asyncContext_ likewise.
• In Proxy.revocable: change the outer _p_ to _proxy_.

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Possible wording for editorial convention:

Every Abstract Closure (AC) is defined within an 'outer' algorithm. Inside the AC, the only outer aliases that are visible are the ones explicitly captured in the AC's header. Any other outer aliases cannot be referenced within the AC. Since these aliases are inaccessible, it might seem that their names could be re-used within the AC (e.g., for a parameter or let-alias), but they should not be re-used, for the sake of clarity.

or:

The body of an Abstract Closure (AC) must not reference aliases of the outer algorithm, other than those explicitly captured in the AC's header. Moreover, for clarity, the AC should not re-use the names of outer aliases for its own purposes (even though those aliases are inacessible).

[bakkot]

lgtm pending renames; I don't have strong feelings about the names. Michael wants to avoid prom style stuff, but both single letter and ac-prefixed names are fine.

[jmdyck]

Michael wants to avoid prom style stuff, but both single letter and ac-prefixed names are fine.

Done.