added at_latest (#900)

* added at_latest

* change some documentation

* remove inline

subxt/src/events/events_client.rs

@@ -37,6 +37,19 @@ where
     /// runtime upgrade. You can attempt to retrieve events from older blocks,
     /// but may run into errors attempting to work with them.
     pub fn at(
+        &self,
+        block_hash: T::Hash,
+    ) -> impl Future<Output = Result<Events<T>, Error>> + Send + 'static {
+        self.at_or_latest(Some(block_hash))
+    }
+
+    /// Obtain events at the latest block hash.
+    pub fn at_latest(&self) -> impl Future<Output = Result<Events<T>, Error>> + Send + 'static {
+        self.at_or_latest(None)
+    }
+
+    /// Obtain events at some block hash.
+    fn at_or_latest(
         &self,
         block_hash: Option<T::Hash>,
     ) -> impl Future<Output = Result<Events<T>, Error>> + Send + 'static {