Introduce iterator trait: IsStateful

[jakobnissen]

Currently, Julia generally assumes iterators are stateless. This is clearly wrong, and so leads to issues:

• Bug: Iterators.cycle does not work for stateful iterators #43235
• collect + Iterators.Stateful + Base.Generator lose last element #42107
• Problematic interaction between Stateful and map #42168
• isempty(::EachLine) expends a line #27412
• Reported length of zip of stateful iterators wrong #47790
• Calling Iterators.takewhile(predicate, itr::Stateful) causes the first non-matching element to be dropped from the Stateful Iterator #48195

I have run into some of these issues myself, and they cause bugs that are quite annoying to track down.
Also, a few days ago, @kescobo reported this:

julia> lines = (i for i in eachline(IOBuffer("a\nb")));

julia> iterate(lines)
("a", nothing)

julia> isempty(lines)
false

julia> iterate(lines)[1]
ERROR: MethodError: no method matching getindex(::Nothing, ::Int64)

Previously, these issues has been resolved by the band-aid approach: Wait until the erroneous assumption that iterators are stateless causes problems, then special-case those problems.
I think @kescobo 's issue is a good example why this is not viable: The number of possible stateful iterators are infinite. We need a more principled approach.

I propose we make a trait that tells if an iterator has state or not. It defaults to IsStateless. We implement IsStateful for a few basic stateful iterators, and then iterators built on top of other stateful iterators are also stateful.
We can then make functions like isempty error for stateful iterators, unless isdone has been defined.

@oxinabox proposed something similar in #27412.