Use copyto! in converting Diagonal/Bidiagonal/Tridiagonal to Matrix

[jishnub]

With this, we may convert a structured matrix to Matrix even if its eltype doesn't support zero(T), as long as we may index into the matrix and the elements have zero defined for themselves. This makes the following work:

julia> D = Diagonal(fill(Diagonal([1,3]), 2))
2×2 Diagonal{Diagonal{Int64, Vector{Int64}}, Vector{Diagonal{Int64, Vector{Int64}}}}:
 [1 0; 0 3]      ⋅     
     ⋅       [1 0; 0 3]

julia> Matrix{eltype(D)}(D)
2×2 Matrix{Diagonal{Int64, Vector{Int64}}}:
 [1 0; 0 3]  [0 0; 0 0]
 [0 0; 0 0]  [1 0; 0 3]

We also may materialize partly initialized matrices:

julia> D = Diagonal(Vector{BigInt}(undef, 2))
2×2 Diagonal{BigInt, Vector{BigInt}}:
 #undef    ⋅
   ⋅     #undef

julia> Matrix{eltype(D)}(D)
2×2 Matrix{BigInt}:
 #undef    0
   0     #undef

The performance seems identical for numeric matrices.