RFC: make CartesianIndex iterable

[darwindarak]

No description provided.

[timholy]

Thanks! But please see discussion in #23719.

[darwindarak]

Oops, didn't see that!

[timholy]

No problem, I think this is at least the third such PR. I'm beginning to wonder if we should do this:

julia> Base.start(::CartesianIndex) = error("I... is deliberately unsupported for CartesianIndex. Use I without the dots or use Tuple(I)...")

julia> for I in CartesianRange(1:5)
           t = (I...)
           @show t
       end
ERROR: I... is deliberately unsupported for CartesianIndex. Use I without the dots or use Tuple(I)...
Stacktrace:
 [1] append_any(::CartesianIndex{1}, ::Vararg{CartesianIndex{1},N} where N) at ./essentials.jl:385
 [2] macro expansion at ./REPL[7]:2 [inlined]
 [3] anonymous at ./<missing>:?