add Lwt_result

[c-cube]

See #226 and #242
This is very basic, I can expand as required by nitpickers ;)

[hcarty]

We should have Lwt.return_ok and Lwt.return_error, similar to Lwt.return_some.

[hcarty]

I'm worried this name may be a little confusing when seen with Lwt.fail. Would error or maybe return_error be clearer?

[c-cube]

I'm considering adding something like

val get_exn : ('a, exn) t -> 'a Lwt.t

to go with catch : 'a Lwt.t -> ('a, exn) t.
Any opinion?

[hcarty]

get_exn 👍

[c-cube]

@Drup proposes to deprecate Lwt.make_value and Lwt.make_error. I have no opinion on that, what do you decide, @aantron?

[aantron]

I think we should deprecate them, in agreement with @Drup.

[aantron]

Thanks @c-cube, @hcarty.

As agreed, we can make modifications in follow-on PRs.