fix shift issue when n character

[jangorecki]

closes #3354
defines shift in data.table.h file
because of that had to rename shift variable in gsum.c, it is now called bitshift.

[HughParsonage]

I could reproduce the segmentation fault outside of RStudio so maybe it should just refer to a segmentation fault rather than suggesting it's something peculiar to RStudio?

[mattdowle]

dt[, new_col := shift(val, NA_integer_)] was still crashing; fixed.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e043dd2). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #3355   +/-   ##
=========================================
  Coverage          ?   94.74%           
=========================================
  Files             ?       65           
  Lines             ?    12154           
  Branches          ?        0           
=========================================
  Hits              ?    11515           
  Misses            ?      639           
  Partials          ?        0

Impacted Files	Coverage Δ
src/shift.c	99.16% <100%> (ø)
R/shift.R	94.11% <100%> (ø)

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e043dd2). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #3355   +/-   ##
=========================================
  Coverage          ?   94.74%           
=========================================
  Files             ?       65           
  Lines             ?    12154           
  Branches          ?        0           
=========================================
  Hits              ?    11515           
  Misses            ?      639           
  Partials          ?        0

Impacted Files	Coverage Δ
src/shift.c	99.16% <100%> (ø)
R/shift.R	94.11% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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