AbstractArray constructor for OneElement

[jishnub]

On master

julia> convert(AbstractArray{Float64}, OneElement(2, 4))
4-element Vector{Float64}:
 0.0
 1.0
 0.0
 0.0

This PR

julia> convert(AbstractArray{Float64}, OneElement(2, 4))
4-element OneElement{Float64, 1, Tuple{Int64}, Tuple{Base.OneTo{Int64}}}:
  ⋅ 
 1.0
  ⋅ 
  ⋅ 

[codecov]

Codecov Report

Merging #261 (e0713bb) into master (91f9f6f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files           5        5           
  Lines         771      772    +1     
=======================================
+ Hits          769      770    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
src/oneelement.jl	100.00% <100.00%> (ø)

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[dlfivefifty]

I think this counts as a "new feature" and warrants a v1.2 tag?

[dlfivefifty]

I'll merge your other PRs after this one and they can all go in v1.2