Currently, block diagonal matrices don't correctly support istril or istriu

julia> D = Diagonal([[1 2; 3 4], [5 6; 7 8]])
2×2 Diagonal{Array{Int64,2},Array{Array{Int64,2},1}}:
 [1 2; 3 4]      ⋅     
     ⋅       [5 6; 7 8]

julia> A = [1 2 0 0; 3 4 0 0; 0 0 5 6; 0 0 7 8] # full matrix of D
4×4 Array{Int64,2}:
 1  2  0  0
 3  4  0  0
 0  0  5  6
 0  0  7  8

julia> istriu(D)
true

julia> istriu(A)
false

These can be solved by adding

istril(D::Diagonal{<:Number}) = true
istril(D::Diagonal) = all(istril, D.diag)

etc.

Once this is fixed, what should the intended behavior of tril(D, k), triu(D, k), diag(D, k), etc. be?
tril(D, k) can just recursively check the blocks but should we make diag(D, k) actually return the k-th super diagonal of the 'full' matrix?

I realize this is quite low on the priority list, but it seems like block diagonal matrices were hastily added and a lot of support is missing.