Tweak unidirectional binding protocols.

[andersio]

1. Replace the observe(_:during:) requirement in the BindingSource protocol with a SignalProducer representation.

   If Generic n-ary producer operators for n > 1. #351 is accepted, BindingSource would adopt arbitrary constraint on associated types and conditional conformances in Swift 4:

   public protocol BindingSource: SignalProducerRepresentable where Error == NoError {}
   extension SignalProducer: BindingSource where Error == NoError {}
   // ... etc.

   While this is breaking, observe(_:during:) is not expected to be publicly used.

2. Move <~ into BindingTargetProvider.

   The SignalProducer representation and the BindingTarget are sufficient points of customisation, and apparently there would not be an alternative semantic of <~ to override for in the context of these two protocols.