Parametrize Triangular on matrix type

[andreasnoack]

and some minor update of Triangular functionality. This should also address #7050

cc:@dmbates, @jiahao

[andreasnoack]

@jiahao Please have a look at this and see if it can be merged (before you refactor the linalg tests again 😡)?

[andreasnoack]

Last call. I'll merge tomorrow if I don't hear any objections.

[dmbates]

Looks good to me. I like this change and the way it can utilize multiple dispatch so effectively.

[andreasnoack]

@jiahao @JeffBezanson I have made the proposed change to zeros(AbstractArray) and added specialised fill! methods to Triangular and Diagonal so now you can do

julia> zeros(Diagonal(randn(4)))
4x4 Diagonal{Float64}:
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0

julia> zeros(Triangular(randn(4,4), :L))
4x4 Triangular{Float64,Array{Float64,2},:L,false}:
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0

I would like to think a little more about setindex! for the special matrix types.