Add Lwt_result.collect without deps

[seprov]

What

This PR adds Lwt_result.collect without adding ocaml-containers as a dependency.

Why

To handle the common case where a list of results need to be collected to a single meaningful result.

Notes

partition_filter_map and split_result are available in ocaml-containers, but to avoid adding the dependency, I've recreated them here.

Also, I'm not sure how y'all like to mention future releases in documentation comments. Let me know about that, thanks.

[raphael-proust]

On a quite specific note: This seems like a simple yet costly way to implement collect. E.g., the oks list is reversed (line 110) whether there are errors or not.

On a more general note: the fact that the Lwt part and the result part of this function are entirely independent makes me wonder whether it actually belongs in Lwt.

[seprov]

Thank you for the feedback.

This seems like a simple yet costly way to implement collect

You're right. If we wanted to add collect to Lwt_result, then I'd be happy to optimize this.

the fact that the Lwt part and the result part of this function are entirely independent makes me wonder whether it actually belongs in Lwt

You're also right that the Lwt and result parts are distinct. If, for example, collect were in CCResult as

let collect x =
  let oks, errors = split_result x in
  match errors with [] -> Ok oks | _ -> Error errors

then usage might look like

my_data_list
|> request_for_each
|> Lwt.all |> Lwt.map CCResult.collect

which isn't much different from or less convenient than

my_data_list
|> request_for_each
|> Lwt_result.collect

If you think that, even with a more efficient implementation of result list filtering and partitioning, this code does not belong in Lwt, then I'd be happy to close this PR.