Add trunc.IDate

[artemklevtsov]

First implementation with some POSIXlt calls.
Should closes #4335.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.47%. Comparing base (b1b1832) to head (13dd733).
⚠️ Report is 1945 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4868      +/-   ##
==========================================
- Coverage   99.61%   99.47%   -0.15%     
==========================================
  Files          72       73       +1     
  Lines       13916    14570     +654     
==========================================
+ Hits        13862    14493     +631     
- Misses         54       77      +23     

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[MichaelChirico]

I would make this more flexible than cut.Date(), though I'm not sure the exact signature that's most helpful.

Is this too flexible?

week_start = "Mon"

offset = if (is.numeric(week_start)) (week_start + 2L) %% 7L else switch(week_start,
  M = , Mon = ,  Monday = 4L,
  Tu = , Tue = ,  Tuesday = 5L,
  W = , Wed = , Wednesday = 6L,
  Th = , Thu = , Thursday = 0L,
  F = , Fri = , Friday = 1L,
  Sa = , Sat = , Saturday = 2L,
  Su = , Sun = , Sunday = 3L)

[MichaelChirico]

Let's make this behave like round.IDate() (for truncation instead of rounding) once #6024 is resolved.