Make $xml_find_function_calls() land on parent::expr instead

[MichaelChirico]

Closes #2431.

[MichaelChirico]

@AshesITR / @IndrajeetPatil PTAL at this today if you can. I need to submit an update to CRAN by tomorrow.

If you prefer, I can just put a small patch on CRAN to meet the deadline & allow a little longer for review here.

[AshesITR]

The change is okay for me on the grounds that we could remove many more parent::expr than we had to add SYMBOL_FUNCTION_CALL.

Have you checked which of self::expr and self::* is faster? I assume it's self::* which would justify its usage despite being slightly less clear IMO.

[MichaelChirico]

I'm 90% sure we've benchmarked * to be faster, but it's worth double-checking. Not worth pausing the PR over, though: #2718