Filter one or multiple events by type from an EventSubscription (#461)

* Split events.rs into multiple files and start work on FilterEvents

* First pass filtering event(s)

* Tweak event examples to show filter_events

* cargo clippy + fmt

* consistify and tidy

* cargo fmt

* Tweak a couple of comments

* Expose phase and block_hash of filtered events, too

* cargo fmt

* expose FilteredEventDetails

* Add docs

* cargo clippy

* remove FilterEvents knowledge of EventSubscription so it's easier to unit test

* unit test filter_events

* tweak an integration test to use filter_events

* cargo fmt

* cargo clippy

* Tweak a comment

Co-authored-by: David <dvdplm@gmail.com>

Co-authored-by: David <dvdplm@gmail.com>

subxt/tests/integration/storage.rs

@@ -28,7 +28,9 @@ use sp_keyring::AccountKeyring;
 async fn storage_plain_lookup() -> Result<(), subxt::Error<DispatchError>> {
     let ctx = test_context().await;
 
-    // Look up a plain value
+    // Look up a plain value. Wait long enough that we don't get the genesis block data,
+    // because it may have no storage associated with it.
+    async_std::task::sleep(std::time::Duration::from_secs(6)).await;
     let entry = ctx.api.storage().timestamp().now(None).await?;
     assert!(entry > 0);