Broadcasting * with Diagonal should return a Diagonal (similarly with Bidiagonal/Tridiagonal)

[dlfivefifty]

The following should take advantage of sparsity:

julia> (1:4) .* Diagonal(1:4)
4×4 Matrix{Int64}:
 1  0  0   0
 0  4  0   0
 0  0  9   0
 0  0  0  16

julia> (1:4) .* Bidiagonal(1:4,1:3,:L)
4×4 Matrix{Int64}:
 1  0   0   0
 2  4   0   0
 0  6   9   0
 0  0  12  16

julia> (1:4) .* Tridiagonal(1:3,1:4,1:3)
4×4 Matrix{Int64}:
 1  1   0   0
 2  4   4   0
 0  6   9   9
 0  0  12  16

I found a related proposal here: JuliaLang/julia#24317

[max-vassili3v]

The PR is here: #1549