Specialize sqrt and cbrt

[jishnub]

After this,

julia> F = Fill(4, 4, 4)
4×4 Fill{Int64}, with entries equal to 4

julia> sqrt(F)
4×4 Fill{Float64}, with entries equal to 1.0

julia> sqrt(F)^2 == F
true

julia> F = Fill(-8, 8, 8)
8×8 Fill{Int64}, with entries equal to -8

julia> cbrt(F)
8×8 Fill{Float64}, with entries equal to -0.5

julia> cbrt(F)^3 == F
true

cbrt doesn't work with complex values because which the definition isn't obvious (JuliaLang/julia#36534).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (9868632) to head (2df5678).
Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #379   +/-   ##
=======================================
  Coverage   99.90%   99.91%           
=======================================
  Files           8        8           
  Lines        1110     1136   +26     
=======================================
+ Hits         1109     1135   +26     
  Misses          1        1           

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[jishnub]

Good to merge? I'm not sure why the codecov job is stuck