Rename flatMap(_:transform:) to flatMap(_:_:).

[andersio]

While the API guidelines says all argument labels should be named unless otherwise specified, the transforms of FP-borrowed operators usually do not have a label given the "terms of art" treatment. So the PR proposes to strip the label from flatMap.

[mdiep]

Something like return self.flatMap(.merge, SignalProducer.init) seems a little weird to me, but I think I'm 👍 on this. 😕

On the one hand, Swift's flatMap has an unnamed argument (and so does our flatMapError). On the other hand, this flatMap has 2 arguments.

Since I don't think this is obviously the better option, I'd like to give @ReactiveCocoa/reactiveswift a chance to speak up in case anyone has an objection.

[mdiep]

No objects, so I think this is good to go. Want to fix up the conflicts?

I'm thinking that we should really make our deprecations file based on the version. So Deprecations+Removals-1.swift, Deprecations+Removals-2.swift, etc.

[andersio]

Rebased.