Broadcasting rules for Diagonal AbstractFill

[jishnub]

Define broadcasting rules so that structure-preserving operations involving a Diagonal AbstractFill do not allocate.

After this PR

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

julia> D .+ D
3×3 Diagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
 4  ⋅  ⋅
 ⋅  4  ⋅
 ⋅  ⋅  4

julia> D .* 2
3×3 Diagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
 4  ⋅  ⋅
 ⋅  4  ⋅
 ⋅  ⋅  4

julia> D .^ 2
3×3 Diagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
 4  ⋅  ⋅
 ⋅  4  ⋅
 ⋅  ⋅  4

[codecov[bot]]

Codecov Report

Merging #165 (bb36731) into master (aeb0aef) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   96.22%   96.29%   +0.06%     
==========================================
  Files           4        4              
  Lines         636      647      +11     
==========================================
+ Hits          612      623      +11     
  Misses         24       24              

Impacted Files	Coverage Δ
src/FillArrays.jl	94.76% <100.00%> (ø)
src/fillbroadcast.jl	97.74% <100.00%> (+0.20%)	:arrow_up:

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[dlfivefifty]

Please add tests for non-structure preserving like D .+ 1

[dlfivefifty]

@jishnub do we want to finish this?

[jishnub]

Perhaps this needs a re-think. I'll see if this can be done in LinearAlgebra. Closing this for now. It's not breaking, so if needed, it can be opened again post 1.0