specialize one for FillArrays

[jishnub]

Fixes #173.
Now

julia> one(Ones(3,3))
3×3 Eye{Float64}

julia> one(Zeros(3,3))
3×3 Eye{Float64}

julia> one(Eye(3))
3×3 Diagonal{Float64, Fill{Float64, 1, Tuple{Base.OneTo{Int64}}}}:
 1.0   ⋅    ⋅ 
  ⋅   1.0   ⋅ 
  ⋅    ⋅   1.0

[codecov]

Codecov Report

Merging #184 (164b0c7) into master (6ed6ec5) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   96.95%   96.97%   +0.02%     
==========================================
  Files           4        4              
  Lines         657      662       +5     
==========================================
+ Hits          637      642       +5     
  Misses         20       20              

Impacted Files	Coverage Δ
src/FillArrays.jl	95.63% <100.00%> (+0.06%)	⬆️

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[dlfivefifty]

Should this be considered breaking?

I think not since it's more of a "bug fix"

[jishnub]

Shouldn't be breaking, since the contract of one is that the result satisfies A * one(A) == one(A) * A == A, which is unchanged. The output of one is not guaranteed to be mutable, or of a specific type.

[cossio]

Why one(Eye(3)) is not Eye(3)?

[dlfivefifty]

Can you make a PR?