Add SignalProducer.init overloads for NoError

[ikesyo]

See Carthage/Carthage#2077 (comment) and Carthage/Carthage#2077 (comment).

Checklist

• [ ] Updated CHANGELOG.md.

[mdiep]

I don't understand what this adds. Could you elaborate? It seems like this would just increase conflicts and require more type annotations.

[ikesyo]

To disambiguate something like this:

let producer1 = SignalProducer(value: "")
	.flatMap(.concat) { str -> SignalProducer<Int, AnyError> in .empty }

let producer2 = SignalProducer(["", ""])
	.flatMap(.concat) { str -> SignalProducer<Int, AnyError> in .empty }

[andersio]

This should be a QoL improvement too, since constant producer now gets NoError inferred by default.

[mdiep]

This breaks source compatibility, but maintains binary compatibility, correct?

[andersio]

I am not convinced that it is source breaking.