Specialize diag for infinite Diagonal

[jishnub]

Address the specific example from #147

After this,

julia> D = Diagonal(1:∞);

julia> diag(D)
1:∞

julia> diag(D,1)
ℵ₀-element Zeros{Int64, 1, Tuple{InfiniteArrays.OneToInf{Int64}}} with indices OneToInf()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9360eb6) 85.92% compared to head (40470cc) 86.02%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
+ Coverage   85.92%   86.02%   +0.09%     
==========================================
  Files           6        6              
  Lines         732      737       +5     
==========================================
+ Hits          629      634       +5     
  Misses        103      103              

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[dlfivefifty]

cool that it's type stable for constants!