Support Diagonal with lazy diag

[putianyi889]

Fixes #31

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.31%. Comparing base (93e155d) to head (3ad26c4).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   69.84%   70.31%   +0.47%     
==========================================
  Files           1        1              
  Lines          63       64       +1     
==========================================
+ Hits           44       45       +1     
  Misses         19       19              

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