RFC: evaluate for iterator in outer scope

[JeffBezanson]

Since it's scope season, here's another issue that, in a way, is the flip side of #22659.

Reasoning by the following transformation:

for i in itr
    body
end

to

foreach(i->body, itr)

itr should be evaluated in the surrounding scope. However, we currently enclose the entire for loop in a new scope, in addition to the per-iteration scope. I believe this is unnecessary.

The breakage here is extremely minor, since for this to matter you basically have to write for i in (a = b), which nobody ever does. Code that does do that is likely to keep working, since you can still see a inside the loop.

[StefanKarpinski]

Itchy trigger-finger, huh? But it's a good change, so 👍 .

[JeffBezanson]

I have a follow-up that conflicts with it :)