LinAlg: fzeropreserving unit triangular broadcast preserves structure

[jishnub]

On master

julia> UU = UnitUpperTriangular(reshape([1:9;],3,3))
3×3 UnitUpperTriangular{Int64, Matrix{Int64}}:
 1  4  7
 ⋅  1  8
 ⋅  ⋅  1

julia> UU .* 2
3×3 Matrix{Int64}:
 2  8  14
 0  2  16
 0  0   2

This PR

julia> UU .* 2
3×3 UpperTriangular{Int64, Matrix{Int64}}:
 2  8  14
 ⋅  2  16
 ⋅  ⋅   2

This also improves performance, as the materialize skips the structured zeros.

julia> UU = UnitUpperTriangular(rand(100, 100));

julia> @btime $UU .* 2;
  12.788 μs (3 allocations: 78.20 KiB) # master
  7.821 μs (3 allocations: 78.20 KiB) # PR