Forward Diagonal rmul!/lmul! for adj/trans to parent

[jishnub]

Currently. rmul!(A'::Adjoint, D::Diagonal) loops over CartesianIndices(A'), which is not cache-friendly. In this PR, we use the fact that A' = A' * D may be rewritten as A = D' * A, and that D' isa Diagonal by design. The operation therefore becomes lmul!(D'::Diagonal, A), which is cache-friendly.
On master,

julia> using LinearAlgebra, Chairmarks

julia> D = Diagonal(rand(4000));

julia> A = rand(size(D)...);

julia> @b (A', D) rmul!(_[1], _[2])
103.276 ms

whereas, with this PR,

julia> @b (A', D) rmul!(_[1], _[2])
9.994 ms

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.00%. Comparing base (f781708) to head (a9ee0f5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1229      +/-   ##
==========================================
+ Coverage   91.93%   92.00%   +0.06%     
==========================================
  Files          34       34              
  Lines       15392    15400       +8     
==========================================
+ Hits        14151    14168      +17     
+ Misses       1241     1232       -9     

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