Fix header visibility.

I had some trouble with ReactiveCocoaIO's iOS target and after some digging I found out `RACSignal+Operations.h` wasn't being exported in 3.0 anymore. The change happened in #877 so I'm pretty sure it was unintentional.

While I was at it I tried to tidy up some private headers being exported and some public ones missing from some of the targets, I split it up in many commits so it's easier to review.