Add invisible param to export attribute

[renkun-ken]

Closes #1024.

This PR add invisible param to export attribute so that if Rcpp::export(invisible=true) is specified, the R wrapper will use invisible(.Call(...)) instead of .Call(...).

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog
• The corresponding Rcpp Attributes vignette is updated accordingly.

[eddelbuettel]

Looks fine in general, but I would of course like to know what @jjallaire thinks here.

I also added an item to the checklist to make sure the corresponding vignette reflects it.

[renkun-ken]

I don't see any unit test about export params. Should I add tests about [[ Rcpp::export(invisible=true) ]]?

[eddelbuettel]

Good question. I don't think we have any, and should maybe table it. Would be nice to ascertain that we can turn visible on/off, that we can renamed (test could use exists() after the build I suppose) but less sure about the RNG option.

Your PR is fine by me now. I will just wait for @jjallaire to glance at it too.

So thanks for the PR and the quick refinements!

[renkun-ken]

Thanks for your kind suggestions!

[kevinushey]

nit: please avoid whitespace-only changes when possible; consider configuring your editor to only trim whitespace on the lines you've edited

[eddelbuettel]

That hit me the other day too. So I just googled it for The One True Editor: https://www.emacswiki.org/emacs/DeletingWhitespace#toc11

Will give that a spin myself.

[renkun-ken]

Sorry for not noticing the auto formatting. I'll disable auto formatting for the project.

[kevinushey]

LGTM!

[jjallaire]

LGTM as well!