AllIndices type for identity indexing

[ChrisRackauckas]

With the proposal for #22858, I've been thinking it could be useful to have a type which commonly represents "AllIndices". So A[AllIndices()] would return A, making it the identity indexing element in some sense.

The reason is so that this could handle non-standard indexing. In some algorithms you might want the user to specify some indices that they "want to save", and you can default to AllIndices. In many of these cases, you haven't built the array yet and may be getting a non-standard type from the user, so defaulting to eachindex or something like that isn't possible. Currently a way to handle this is to default the chosen indices to nothing, and if you see nothing you treat it as AllIndices applied to the type by iterating eachindex after the array is built.

The name could change of course.

[mbauman]

How is this different from A[:]? Edit: duh, A[:] will reshape to a vector.

[ChrisRackauckas]

Colon doesn't keep shape:

julia> A = rand(2,2)
2×2 Array{Float64,2}:
 0.138581  0.120538
 0.528131  0.11805

julia> A[:]
4-element Array{Float64,1}:
 0.138581
 0.528131
 0.120538
 0.11805

[mbauman]

This could be solved with #5405 as a base case (just the ellipsis).

[mbauman]

Now this is a duplicate of #5405. A single ellipsis represents this operation well, and it's now in EllipsisNotation.jl as A[..].