Add a fully typed Diagonal constructor from AbstractMatrixes

[jishnub]

The following works after this PR:

julia> oftype(Diagonal(Float32[1,2]), [1 0; 0 2])
2×2 Diagonal{Float32, Vector{Float32}}:
 1.0   ⋅ 
  ⋅   2.0

This changes the behavior of the constructor to copy the diagonal, so now

julia> D = Diagonal([1,2]);

julia> typeof(D)(D).diag === D.diag
false

whereas this used to be true previously. This probably doesn't matter much, as in most non-trivial cases it'd be copied anyway, and this conversion is unusual in the trivial case.

[jishnub]

The test failure appears to be unrelated

[jishnub]

Good to go?

[dkarrasch]

One last comment, otherwise let's go.