[RFC] Add eachrsplit iterator

[jakobnissen]

Unlike rsplit, this iterator returns split substrings right to left, but other- wise it behaves just like eachsplit.
This design has been chosen to avoid either a costly double traversal of the input string, or needing a stack to store the strings. Both of these workarounds would lessen the appeal compared to simply using rsplit.

Closes #45385

Request for comments

• Is it acceptable that it returns the substrings in reverse order compared to rsplit? I believe this is unfortunately necessary.

[jakobnissen]

Squashed, otherwise unchanged. I think the ASAN build failure is unrelated to this PR.

[stevengj]

Is there a reason why this wasn't implemented as an Iterators.Reverse{SplitIterator}, so that you could just do Iterators.reverse(eachsplit(...)), and instead introduced a completely new iterator type and API?

[vtjnash]

IIRC, this isn't just Iterators.reverse(eachsplit) because of the limit parameter

[stevengj]

I see. So we could implement something like:

function Iterators.reverse(itr::Union{SplitIterator,RSplitIterator})
    itr.limit <= 0 || throw(ArgumentError("reverse does not support split iterators with limit = $(itr.limit) > 0"))
    return (itr isa SplitIterator ? RSplitIterator : SplitIterator)(itr.str, itr.splitter, itr.limit, itr.keepempty)
end

[vtjnash]

No, even then it is not the same, here's another counter-example:

julia> eachrsplit("abbbcbbbdf", "bb") |> collect
3-element Vector{SubString{String}}:
 "df"
 "cb"
 "ab"

julia> eachsplit("abbbcbbbdf", "bb") |> collect
3-element Vector{SubString{String}}:
 "a"
 "bc"
 "bdf"