Avoid materializing diag in Diagonal kron

[jishnub]

This reduces allocations in kron

julia> using LinearAlgebra

julia> D = Diagonal(1:100);

julia> T = Tridiagonal(1:99, 1:100, 1:99);

julia> @b (D,T) kron(_[1], _[2])
14.162 μs (20 allocs: 239.164 KiB) # master
8.829 μs (14 allocs: 236.445 KiB) # this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.97%. Comparing base (508e77f) to head (9a265a8).
Report is 32 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1230   +/-   ##
=======================================
  Coverage   91.97%   91.97%           
=======================================
  Files          34       34           
  Lines       15400    15400           
=======================================
  Hits        14164    14164           
  Misses       1236     1236           

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[ViralBShah]

Ok to backport to 1.12?

[jishnub]

Yes, should be ok