Support broadcasting over structured block matrices (#53909)
Fix https://github.com/JuliaLang/julia/issues/48664
After this, broadcasting over structured block matrices with
matrix-valued elements works:
```julia
julia> D = Diagonal([[1 2; 3 4], [5 6; 7 8]])
2×2 Diagonal{Matrix{Int64}, Vector{Matrix{Int64}}}:
[1 2; 3 4] ⋅
⋅ [5 6; 7 8]
julia> D .+ D
2×2 Diagonal{Matrix{Int64}, Vector{Matrix{Int64}}}:
[2 4; 6 8] ⋅
⋅ [10 12; 14 16]
julia> cos.(D)
2×2 Matrix{Matrix{Float64}}:
[0.855423 -0.110876; -0.166315 0.689109] [1.0 0.0; 0.0 1.0]
[1.0 0.0; 0.0 1.0] [0.928384 -0.069963; -0.0816235 0.893403]
```
Such operations show up when using `BlockArrays`.
The implementation is a bit hacky as it uses `0I` as the zero element in
`fzero`, which isn't really the correct zero if the blocks are
rectangular. Nonetheless, this works, as `fzero` is only used to
determine if the structure is preserved.
