Non-numeric eltypes in OneElement

[jishnub]

This lifts the somewhat unnecessary restriction to numeric eltypes in the OneElement constructor. After this, the following works:

julia> OneElement(SMatrix{2,2}(1:4), (2,2), (2,2))
2×2 OneElement{SMatrix{2, 2, Int64, 4}, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}:
     ⋅           ⋅     
     ⋅       [1 3; 2 4]

In general, indexing into this requires that the element type defines zero(T) and ideally returns a value of type T.

Also, fixes some ambiguities in the 0-D Fill and OneElement constructors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1a9746c) 99.88% compared to head (d7eb614) 99.88%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #315   +/-   ##
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  Coverage   99.88%   99.88%           
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  Files           8        8           
  Lines         906      906           
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  Hits          905      905           
  Misses          1        1           

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