Increase the flexibility of unfold

[aweinstock314]

16:58:55 <aweinstock> Is there a compelling reason for unfold's step function to be (State -> Option<IntoFuture<Item=(StreamItem, State)>> rather than (State -> IntoFuture<Item=Option<(StreamItem, State)>>)?
16:59:43 <aweinstock> The flexibility to choose whether or not to yield the next item in the continuation of a future is helpful (perhaps necessary?) in some cases
16:59:58 <acrichto> hm yeah good point, makes sense to me!
17:00:02 <acrichto> want to open an issue w/ that?
17:00:12 <acrichto> unfortunately it'll have to wait until 0.2 at this point, but I can be sure we include it
17:00:32 <aweinstock> will do
17:00:50 <aweinstock> (I'm currently using my own version of unfold in https://github.com/aweinstock314/kingr4_weinsa_distributedsystems_project2/blob/master/src/main.rs#L302-L322 due to this)
17:01:38 <acrichto> yeah putting the option in the future makes sense to me

[carllerche]

Is there an action item to take from here? This issue has been inactive for over a year now.

[sfackler]

I think the action is to change the signature of unfold in 0.2.

[alexcrichton]

Fixed in #746 (review)