Allow C-level calls to forder

[mattfidler]

Closes #4015

I don't actually think this is a duplicate; It allows calling the underlying C code with evaluating in R, which could be faster.

[codecov]

Codecov Report

Merging #4017 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4017      +/-   ##
==========================================
+ Coverage   99.41%   99.41%   +<.01%     
==========================================
  Files          72       72              
  Lines       13904    13905       +1     
==========================================
+ Hits        13823    13824       +1     
  Misses         81       81

Impacted Files	Coverage Δ
src/init.c	100% <100%> (ø)	⬆️

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[mattdowle]

@lsilvest this PR by @mattfidler exports one more C function. It didn't have the & prefix on the function so I added that to be consistent with your #4084 on the assumption you had tested subsetDT and that's working from your package. Then it seemed cleaner to drop the C prefix in the name since it's already clear from having to call R_GetCCallable that it's a C function that data.table is exporting. So I dropped the C prefix from subsetDT in this PR too. Hope ok.

[MichaelChirico]

need to add to ?cdt as well right

[mattdowle]

Needs addition to the news item about subsetDT, and addition to ?cdt, please.
Also needs a comment adding to the argument list of forder() reminding us that it's exported and we can't make changes to it in future as easily. Like the comment Jan added to subsetDT in #4084 iirc.
Tagged WIP. When it's untagged WIP I'll look again and merge.

I invited you to be project member, @mattfidler. This enabled you to create branches in the main project in future. You'll need to accept the invitation which shows as a button on your GitHub profile or GitHub repository page. Welcome!
Whether adding one line to export a C functions counts as a code contribution for the purpose of adding to contributor list in DESCRIPTION, I'm not sure.

[mattfidler]

Sorry Missed your comment @mattdowle. There is alot of traffic on data.table.

Thank you for the excellent package. I don't need to be a contributor for this (in my opinion). However CRAN may differ from my opinion (they have in the past). I am fine adding myself as a contributor if I need to.

[mattfidler]

I also couldn't see a project member email or any buttons; Perhaps the invitation has expired.

[jangorecki]

@mattfidler are you planning to incorporate the feedback (cdt manual, news file, description file, comment in forder.c)? or you are not interested anymore in completing this PR?

[mattfidler]

I realized this is still open. I had forgot about this