specialize kron!(::Diagonal, ::Diagonal, ::Diagonal)

[tam724]

see #1391.
Implements a specialized version of kron! that dispatches on ::Diagonal as the first argument.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (db95a2a) to head (2ffcafa).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1392   +/-   ##
=======================================
  Coverage   93.83%   93.84%           
=======================================
  Files          34       34           
  Lines       15823    15826    +3     
=======================================
+ Hits        14848    14852    +4     
+ Misses        975      974    -1     

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[dkarrasch]

Since the 2-arg method doesn't redirect to the in-place method, we would need a test for this.

[dkarrasch]

Since we're at it, we might as well "fix" the style bits. Otherwise LGTM.

[tam724]

Thanks for the review. I added the whitespaces.

[dkarrasch]

Not sure why it says those lines are not covered... This LGTM, so I'll merge.

[dkarrasch]

Thank you very much, @tam724!