Explore use of _RO (read-only) SEXP pointers

[MichaelChirico]

Following up from #4508 (comment)

R is also having read-only pointers that are const (e.g. INTEGER_RO -> const int*).

Should we try and adopt using this internally?

Considerations:

• Is this in the API? I don't see any of my other GitHub clones using it
• Is it back-compatible? Need to check r-source history
• Is there any measurable payoff?

Basic look at the potential effort for combing over the current source:

SEXPTYPE	count in src
LOGICAL	161
INTEGER	424
REAL	189
COMPLEX	48

Details

code to generate table:

echo "| \`SEXPTYPE\` | count in \`src\` |" && \
echo "| ---------- | -------------- |" && \
for type in LOGICAL INTEGER REAL COMPLEX
do echo "| $type |" $(grep -Fr "$type(" src | wc -l) "|"
done

[MichaelChirico]

Right away I notice they're not available before 3 years ago:

wch/r-source@4b7335d

That's roughly R 3.4.4 by my reckoning so I don't think we'll have those available in the foreseeable future. Is it still worth exploring a performance impact?

FWIW I tried using it in psum branch and didn't notice any huge difference, so closing for now.