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.

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

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

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