DT[i,cols] calls CsubsetDT

[mattdowle]

Closes #3212
Closes #3214

• revisit test 1542 before merge (test was good and is unchanged, dt[TRUE, B := B * 2] allocates 28 * nrow(dt) bytes of data #1249)

Reveals #3215 which is unchanged from v1.11.8 and left open after this PR.

[codecov]

Codecov Report

Merging #3213 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3213      +/-   ##
==========================================
+ Coverage   94.57%   94.59%   +0.02%     
==========================================
  Files          61       61              
  Lines       11737    11735       -2     
==========================================
+ Hits        11100    11101       +1     
+ Misses        637      634       -3

Impacted Files	Coverage Δ
src/subset.c	100% <100%> (ø)	⬆️
R/data.table.R	95.18% <100%> (+0.14%)	⬆️

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[codecov]

Codecov Report

Merging #3213 into master will decrease coverage by 0.06%.
The diff coverage is 86.95%.

@@            Coverage Diff            @@
##           master   #3213      +/-   ##
=========================================
- Coverage   94.57%   94.5%   -0.07%     
=========================================
  Files          61      61              
  Lines       11737   11747      +10     
=========================================
+ Hits        11100   11102       +2     
- Misses        637     645       +8

Impacted Files	Coverage Δ
src/subset.c	100% <100%> (ø)	⬆️
R/data.table.R	94.64% <83.33%> (-0.4%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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