Generalize Diagonal * AdjOrTransAbsMat to arbitrary element types

[jishnub]

The current implementation assumes that the adjoint matrix may be copied to the destination, but this is not necessary in general. This PR changes the implementation to use mul! instead of copy + (l/r)mul! removes the specialized methods, so that the multiplication generalizes to arbitrary element types. In particular, the following works after this:

julia> SM = SMatrix{2,3}(reshape([1:6;],2,3))
2×3 SMatrix{2, 3, Int64, 6} with indices SOneTo(2)×SOneTo(3):
 1  3  5
 2  4  6

julia> D = Diagonal(fill(SM,2))
2×2 Diagonal{SMatrix{2, 3, Int64, 6}, Vector{SMatrix{2, 3, Int64, 6}}}:
 [1 3 5; 2 4 6]        ⋅       
       ⋅         [1 3 5; 2 4 6]

julia> A = fill(SM,2,2)'
2×2 adjoint(::Matrix{SMatrix{2, 3, Int64, 6}}) with eltype SMatrix{3, 2, Int64, 6}:
 [1 2; 3 4; 5 6]  [1 2; 3 4; 5 6]
 [1 2; 3 4; 5 6]  [1 2; 3 4; 5 6]

julia> D * A
2×2 Matrix{SMatrix{2, 2, Int64, 4}}:
 [35 44; 44 56]  [35 44; 44 56]
 [35 44; 44 56]  [35 44; 44 56]

julia> A * D
2×2 Matrix{SMatrix{3, 3, Int64, 9}}:
 [5 11 17; 11 25 39; 17 39 61]  [5 11 17; 11 25 39; 17 39 61]
 [5 11 17; 11 25 39; 17 39 61]  [5 11 17; 11 25 39; 17 39 61]