More robust setDT

[sritchie73]

Fixes #4176

This PR reorganises the code within setDT() so that the checks currently made on list objects (checks for POSIXlt and heterogeneous column lengths) are made regardless of the class of the input, and that setalloccol() is only called once all checks have passed. This means you now can't bypass these checks by simply adding "data.table" or "data.frame" as a class to a list of mixed column lengths. I've also fixed the column length checking code to ignore NULL columns, as per #3480 and #3471, which would have triggered the range-failure check unless all columns were NULL or otherwise length 0.

Ideally it would be nice to be able to move some of these checks into setalloccol(), so that we could use selfrefok(x) > 0L as a way of checking an input object is a non-malformed data.table.

[codecov]

Codecov Report

Merging #4193 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4193      +/-   ##
==========================================
+ Coverage   99.61%   99.61%   +<.01%     
==========================================
  Files          72       72              
  Lines       13871    13880       +9     
==========================================
+ Hits        13817    13826       +9     
  Misses         54       54

Impacted Files	Coverage Δ
R/data.table.R	100% <100%> (ø)	⬆️

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