add support for complex values in value.var in dcast

[MichaelChirico]

Closes #4855

[codecov]

Codecov Report

Merging #4863 (aa87936) into master (3b82d51) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4863   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          76       76           
  Lines       14681    14692   +11     
=======================================
+ Hits        14614    14625   +11     
  Misses         67       67           

Impacted Files	Coverage Δ
src/fcast.c	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3b82d51...aa87936. Read the comment docs.