ToDos: replace looped subsetVector with subsetDT

[ben-schwen]

Towards #678

[codecov]

Codecov Report

Merging #5264 (8c06744) into master (96860f2) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5264      +/-   ##
==========================================
- Coverage   99.50%   99.50%   -0.01%     
==========================================
  Files          77       77              
  Lines       14603    14599       -4     
==========================================
- Hits        14531    14527       -4     
  Misses         72       72              

Impacted Files	Coverage Δ
R/data.table.R	99.89% <100.00%> (-0.01%)	⬇️
R/setops.R	100.00% <0.00%> (ø)

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