Use _RO accessors for const targets

[MichaelChirico]

Searched the sources:

grep -Er "const\b.*[*]\w+\s*=\s*[A-Z]+([^OR]|[^A]R)[(]" src

This is some low-hanging fruit in terms of better use of read-only accessors. There will be other cases where we can make the change to a const pointer as well, but those can't be identified with a simple regex.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (e13ea60) to head (60f2ba2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7611   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          87       87           
  Lines       16903    16903           
=======================================
  Hits        16739    16739           
  Misses        164      164           

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[github-actions]

• HEAD=ro-access stopped early for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 60f2ba2

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	5 minutes and 23 seconds
Installing different package versions	11 minutes and 21 seconds
Running and plotting the test cases	4 minutes and 4 seconds

[MichaelChirico]

Another big class of cases where we could switch to _RO are call sites like INTEGER(x)[k] -- once we do [ to the output, we know we're only reading from x.

There are roughly 240 such sites (grep -Er "(INTEGER|LOGICAL|REAL|COMPLEX|RAW)[(]\w+[)]\[[^=]*$"), I just suspect it's not worth it to change them.

[MichaelChirico]

I see this has some (or total) overlap with #7394. I think it's good to break out the changes into minimally-digestible parts -- here we focus only on const <type> *<var> = <writeable>(x) --> const <type> *<var> = <read-only>(x).