Add some basic Xcode groups
Rename ColdSignal.start to startMulticasted
Rename ColdSignal.subscribe to start
Combine Foundation extensions into one Swift file
Add ColdSignal.takeUntil
Make Action.enabled complete upon deallocation
Generalize ColdSignal.start to any SinkType
Add ColdSignal.concat for signals of signals
Concatenate in buffer() and replay(), don't merge