polish: use closure instead of CollectFieldsContext

[yaacovCR]

While working on #4360, I felt myself trying to reach for opportunity to remove the CollectFieldsContext type/object that I previously introduced; this is accomplished in this separate PR.

Benchmarking on my machine, this seems essentially no better and no worse in the performance realm. I am evaluating this in terms of readability, both in the local sense and in the overall algorithmic sense of better understanding the method and limit of how we are recursing.

Overall, I think it's a minor improvement.

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[yaacovCR]

Note: this would probably be most helpfully reviewed by ignoring whitespace differences and possibly side-by-side comparison.

[yaacovCR]

Also subtracts about 50 lines of code, which is always good.

From my machine:

[yaacovCR]

Also subtracts about 50 lines of code, which is always good.

Mostly whitespace/formatting though, so your mileage may vary, but it helps make the code more readable for me.

[yaacovCR]

closing for now, I suppose it's somewhat helpful to have the arguments for shouldIncludeNode be more explicitly declared (rather than referencing the variables from the enclosure). we can always revisit this if it seems more compelling.