Specialize LinearAlgebra.symmetric for AbstractFill

[jishnub]

This is used in places in LinearAlgebra, and currently

julia> F = Fill(3, 4, 4)                                                          
4×4 Fill{Int64}, with entries equal to 3

julia> LinearAlgebra.symmetric(F)                                                 
4×4 Symmetric{Int64, Fill{Int64, 2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}}:
 3  3  3  3
 3  3  3  3
 3  3  3  3
 3  3  3  3

However, since a Fill is symmetric, we may avoid the wrapper and obtain

julia> LinearAlgebra.symmetric(F)                                                 
4×4 Fill{Int64}, with entries equal to 3

This helps ensure type-stability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (f659d9f) to head (f66544b).
Report is 4 commits behind head on master.

Additional details and impacted files

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##           master     #372   +/-   ##
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  Coverage   99.90%   99.90%           
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  Files           8        8           
  Lines        1106     1108    +2     
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+ Hits         1105     1107    +2     
  Misses          1        1           

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