Add parallel iterators for map.keys and map.values

[jabraham17]

Adds the missing parallel iterators for map.keys() and map.values().

Resolves #24056

• Tested with a full paratest with/without gasnet

Implementation note: this implements the parallel iterators nearly identically to the ones for set.these()

[Reviewed by @lydia-duncan]

[cassella]

Implementation note: this implements the parallel iterators nearly identically to the ones for set.these()

It doesn't look nearly identical to me, maybe because of the changes to set.these() in #26595. (I think these these() will have the same issue those these() had prior to that PR.)

[jabraham17]

That is correct, this PR predates that work and has not been updated to match yet

[cassella]

Sure. I was just making a note since github repored that it wasn't going to cause a merge conflict; the pr description is now stale; and the state of the art in set.these() has advanced.

[lydia-duncan]

Nice, that looks good! To verify it's working the same as the Set improvement, it'd be nice to see:

• A test of zippering the keys of one map with the keys of a separate map of the same length (and/or the same thing but with values)
• A test of zippering either the keys or the values of a map with a set of the same length.

But I have no reason to believe those cases don't work today, since they're benefiting from the same code as Set.

[lydia-duncan]

Awesome, thank you!