Deprecate Lwt_stream.result

[aantron]

And prepare to break Lwt_stream.map_exn, which uses this type.

Users of the current map_exn will see:

Warning 3: deprecated: Lwt_stream.map_exn
This function will soon use Lwt.result instead of Lwt_stream.result.
To continue using the present signature, use Lwt_stream.Versioned.map_exn_1
To use the new variant immediately, use Lwt_stream.Versioned.map_exn_2

cc @c-cube more result meltdown :P

[Drup]

Lwt_stream.Versioned ? shudder a little bit.

That's not a fabulous way of making breaking change. I'm pretty sure you could just come up with a better name (wrap_all, for parallel with Lwt.wrap ?) and make the transition like that.

[vasilisp]

@Drup, see #293 if you haven't already.

[aantron]

@vasilisp, @Drup Thanks. #293 is how I intend to make breaking changes if we want to keep the name.

But, if map_exn is the wrong name for this function to begin with, fair enough. Then, if we agree on a good name for the new version (will take a look at wrap/wrap_exn later, preoccupied with something), I will only deprecate map_exn and the type and move them to the bottom of the API docs, and keep them there for a very long time. No need for .Versioned in that case.

[aantron]

Renamed it to wrap_exn. There is no .Versioned module anymore.