New Event Subscription API (#442)

* Add reworked event types

* first pass implementing event subscriptions

* make clear that some methods are private

* comment tidy

* use Events in transaction stuff

* align transaction and event APIs

* remove __private_ prefixes; they are ugly

* fix examples and remove old events and subscription code

* better comments on hidden event functions

* re-add find_first_event; it's used a bunch in tests and examples

* cargo check --all-targets now passes

* Fix up existing event tests

* cargo fmt

* change todo to note

* clippy and doc niggles

* revert to find_first_event

* Add specific subscription related tests

* cargo fmt

* Update tests and add/fix examples

* cargo fmt

* add a little to subscribe_all_events example

* cargo fmt

* move an example comment

* easy access to root mod for more clarity

* add a couple of tests to ensure that events properly decoded until naff bytes

* Simplify EventSubscription Stream impl a little

* Address some PR feedback

subxt/tests/integration/frame/system.rs

@@ -56,7 +56,7 @@ async fn tx_remark_with_event() -> Result<(), subxt::Error<DispatchError>> {
         .await?
         .wait_for_finalized_success()
         .await?
-        .has_event::<system::events::Remarked>()?;
+        .has::<system::events::Remarked>()?;
 
     assert!(found_event);
     Ok(())