Signal resource management with Lifetime.

[andersio]

Follow-up to #334.

This is not as trivially migratable as #334, since Signal had a different design choice in the signal generator closure than the SignalProducer. Let alone the fact that SE-0110 is not implemented in Swift 3, adding obstacles to automatic migration.

1. Change the Signal generator signature to (Observer, Lifetime) -> Void, so that it matches the style of SignalProducer.init.

   let signal = Signal { observer, lifetime in
      let disposable = self.observe { ... }
      _ = (disposable?.dispose).map(lifetime.observeEnded)
   }
   
   let producer = SignalProducer { observer, lifetime in
      let disposable = producer.start { ... }
      lifetime.observeEnded(disposable.dispose)
   }

[mdiep]

Change the Signal generator signature to (Observer, Lifetime) -> Void, so that it matches the style of SignalProducer.init.

This makes sense to me. (Although I did think about it for quite a while because I wasn't sure.)

Replace pipe(disposable:) with a new variant of pipe returning a 3-tuple (input, output and lifetime).

This change seems less necessary and also orthogonal. I'm not convinced we should do this.

---

Unfortunately, this is all too late for 2.0, so it will also need to be changed to be a non-breaking change (adding an init instead of changing it) or wait for 3.0.

[andersio]

@mdiep Sadly as a new init it would be ambiguous given the tuple splat behaviour. The only option not to wait for 3.0 is exposing it as static function.

[mdiep]

3.0 it is!

Hopefully we/I can be a little more prompt with 3.0. We could ship as soon as Xcode 9 / Swift 4 ship this fall.

[mdiep]

I strongly dislike using map like this. map shouldn't be used for side effects. This is also not very clear IMO.

I think we're better off with if let dispose = disposable?.dispose { lifetime.observeEnded(dispose) }.

[andersio]

lifetime += disposable should work, and I have already replaced most of them.

[mdiep]

This should be mentioned in the changelog, but otherwise it looks great. 👍