Forward structure-preserving broadcasting to diag for Diagonal

[jishnub]

For example, after this PR,

julia> D = Diagonal(1:4)
4×4 Diagonal{Int64, UnitRange{Int64}}:
 1  ⋅  ⋅  ⋅
 ⋅  2  ⋅  ⋅
 ⋅  ⋅  3  ⋅
 ⋅  ⋅  ⋅  4

julia> D .* 2
4×4 Diagonal{Int64, StepRangeLen{Int64, Int64, Int64, Int64}}:
 2  ⋅  ⋅  ⋅
 ⋅  4  ⋅  ⋅
 ⋅  ⋅  6  ⋅
 ⋅  ⋅  ⋅  8

julia> using SparseArrays

julia> D = Diagonal(spzeros(2))
2×2 Diagonal{Float64, SparseVector{Float64, Int64}}:
 0.0   ⋅ 
  ⋅   0.0

julia> D .* 2
2×2 Diagonal{Float64, SparseVector{Float64, Int64}}:
 0.0   ⋅ 
  ⋅   0.0

julia> using FillArrays

julia> D = Diagonal(Fill(3, 2))
2×2 Diagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
 3  ⋅
 ⋅  3

julia> D .* 2
2×2 Diagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
 6  ⋅
 ⋅  6

I've not handled the other banded matrix types in this PR, but Diagonal is probably used far more commonly in packages anyway.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.88%. Comparing base (ed53855) to head (5898006).

Files with missing lines	Patch %	Lines
src/structuredbroadcast.jl	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1423      +/-   ##
==========================================
- Coverage   93.88%   93.88%   -0.01%     
==========================================
  Files          34       34              
  Lines       15891    15899       +8     
==========================================
+ Hits        14920    14927       +7     
- Misses        971      972       +1     

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