Mark Event enum non_exhaustive

Frando · Frando · commit 6b1feefe3db4 · 2020-05-19T12:24:23.000+02:00
... so that addding new events is not a breaking change.
diff --git a/src/event.rs b/src/event.rs
@@ -1,5 +1,6 @@
 /// An event emitted by a Feed.
 #[derive(Debug, Clone, PartialEq)]
+#[non_exhaustive]
 pub enum Event {
     /// A new block has been appended.
     Append,
