2-arg show for Symmetric/Hermitian and triangular

[jishnub]

After this,

julia> A = rand(2,2);

julia> H = Hermitian(A)
2×2 Hermitian{Float64, Matrix{Float64}}:
 0.889108  0.264298
 0.264298  0.234985

julia> println(H)
Hermitian([0.8891078167175059 0.26429820656950753; 0.26429820656950753 0.23498548977257172])

julia> S = Symmetric(A)
2×2 Symmetric{Float64, Matrix{Float64}}:
 0.889108  0.264298
 0.264298  0.234985

julia> println(S)
Symmetric([0.8891078167175059 0.26429820656950753; 0.26429820656950753 0.23498548977257172])

julia> U = UpperTriangular(A)
2×2 UpperTriangular{Float64, Matrix{Float64}}:
 0.889108  0.264298
  ⋅        0.234985

julia> println(U)
UpperTriangular([0.8891078167175059 0.26429820656950753; 0.0 0.23498548977257172])

julia> U = UnitUpperTriangular(A)
2×2 UnitUpperTriangular{Float64, Matrix{Float64}}:
 1.0  0.264298
  ⋅   1.0

julia> println(U)
UnitUpperTriangular([1.0 0.26429820656950753; 0.0 1.0])

julia> L = LowerTriangular(A)
2×2 LowerTriangular{Float64, Matrix{Float64}}:
 0.889108   ⋅ 
 0.2504    0.234985

julia> println(L)
LowerTriangular([0.8891078167175059 0.0; 0.2503997651590879 0.23498548977257172])

julia> L = UnitLowerTriangular(A)
2×2 UnitLowerTriangular{Float64, Matrix{Float64}}:
 1.0      ⋅ 
 0.2504  1.0

julia> println(L)
UnitLowerTriangular([1.0 0.0; 0.2503997651590879 1.0])

The displayed form is now a valid constructor, and may be copy-pasted to reconstruct the object.

[ViralBShah]

Worth getting this in? I assume it is marked Draft because it has more work to be done.

[jishnub]

There were some test failures because of how arrays of arrays would now be printed. I haven't really thought about this since.

[DilumAluthge]

We have moved the LinearAlgebra stdlib to an external repo: https://github.com/JuliaLang/LinearAlgebra.jl

@jishnub If you think that this PR is still relevant, please open a new PR on the LinearAlgebra.jl repo.