Signal.Event: changed to @frozen (#841)

This `enum` isn't expected to change. It's important to mark it as `@frozen`
so that clients don't get warnings whem compiling with `BUILD_LIBRARY_FOR_DISTRIBUTION`,
which might be required in a future version of Carthage supporting Catalyst.
(See Carthage/Carthage#3235).

Sources/Event.swift

@@ -6,6 +6,7 @@ extension Signal {
 	///
 	/// Signals must conform to the grammar:
 	/// `value* (failed | completed | interrupted)?`
+	@frozen
 	public enum Event {
 		/// A value provided by the signal.
 		case value(Value)