docs: Make empty listener panic clearer

This commit makes the panic message for a listener that's not inserted
into the linked list much clearer. The goal is to convey to the user
that they may be `poll`ing the listener after it has completed.

This commit also fixes some new Clippy lints.

cc #124

Signed-off-by: John Nunley <dev@notgull.net>

src/lib.rs

@@ -1248,7 +1248,7 @@ impl<T> RegisterResult<T> {
         match self {
             Self::Notified(tag) => Some(tag),
             Self::Registered => None,
-            Self::NeverInserted => panic!("listener was never inserted into the list"),
+            Self::NeverInserted => panic!("{}", NEVER_INSERTED_PANIC),
         }
     }
 }
@@ -1326,6 +1326,10 @@ impl TaskRef<'_> {
     }
 }
 
+const NEVER_INSERTED_PANIC: &str = "\
+EventListener was not inserted into the linked list, make sure you're not polling \
+EventListener/listener! after it has finished";
+
 /// Synchronization primitive implementation.
 mod sync {
     pub(super) use core::cell;