order optimizes to forderv, uses key or index, forder more consistent to base order

[jangorecki]

Closes:

• DT[order(.)] could utilize existing index order could utilize existing index #3023 (once Teach forder to re-use existing key and index attributes instead of sorting from scratch #4386 merged) - this one and the next, setindex, is being tested on lazy-forder branch after merging this PR in opt-order-lazy branch
• DT[order(.), ...] could create index on DT DT[order(.), ...] could create index on DT #3261 (once Teach forder to re-use existing key and index attributes instead of sorting from scratch #4386 merged, and setindex from C forder enabled)
• order optimization does not recognize all order args properly order optimization does not recognize all order args properly #4456
• provides forder manual, doesn't have to be exported, a lot to migrate from setorder.Rd, partly done in setorderv gets new arg neworder, closes #4012 #4340

To reduce number of API changes (in still internal forder[v]) I am not going to yet close this issue:

• partially: forder should not accept list, but forderv should forder should not accept list, but forderv should #4346

It is partially addressed by making forder compatible with base::order, but this PR does not restrict input types. Lets see what base R is planning to do about order(df) (see R-devel discussion), and in another iteration we can solve #4346 and #4214.

It also makes na.last=NA option in forder consistent to base order by removing 0's that are result from forderv(na.last=NA).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (dd7609e) to head (7791b6e).
⚠️ Report is 1928 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4458      +/-   ##
==========================================
- Coverage   99.61%   99.59%   -0.02%     
==========================================
  Files          72       72              
  Lines       13917    13967      +50     
==========================================
+ Hits        13863    13911      +48     
- Misses         54       56       +2     

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[jangorecki]

[.integer automatically recycle scalar decreasing

[jangorecki]

marking as WIP because it will need rebase over lazy-forder branch