sqrt, cbrt and log for dense diagonal matrices

[jishnub]

This PR improves performance by only applying the functions to the diagonal elements:

julia> A = diagm(0=>ones(100));

julia> @btime log($A);
  364.163 μs (22 allocations: 401.62 KiB) # master
  13.528 μs (7 allocations: 80.02 KiB) # this PR

Similar improvements for sqrt and cbrt as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.89%. Comparing base (39a7e3c) to head (4f02f26).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1156      +/-   ##
==========================================
+ Coverage   91.85%   91.89%   +0.03%     
==========================================
  Files          34       34              
  Lines       15339    15345       +6     
==========================================
+ Hits        14090    14101      +11     
+ Misses       1249     1244       -5     

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