Slightly faster detection of constant columns

[HughParsonage]

No description provided.

[codecov]

Codecov Report

Merging #6 into master will increase coverage by 0.7%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master       #6     +/-   ##
=========================================
+ Coverage   99.02%   99.72%   +0.7%     
=========================================
  Files          27       27             
  Lines         718      738     +20     
=========================================
+ Hits          711      736     +25     
+ Misses          7        2      -5

Impacted Files	Coverage Δ
R/drop_constant_cols.R	100% <100%> (ø)	⬆️
R/select_grep.R	100% <0%> (+6.75%)	⬆️

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