Return events from blocks skipped over during Finalization, too (#473)

* make subscription stream generic in EventSubscription

* rename to EventSub/FinalizedEventSub

* wip fix some lifetimes so that event sub can depend on client in stream

* Cargo fmt + comment tweaks

* Add another comment

* factor out prev block header fetching into a separate function to tidy

* add a comment

* remove ListOrValue as it's unused

* Into<u128> on BlockNumber to simplify things

* clippy

* Fix an example and clippy

* simplify iterator now we are Into<u128>

* Into<u64> instead because it needs serializing, and test core logic

* Tweak missing block test to fill in >=2 holes

* tweak a comment

subxt/src/events/mod.rs

@@ -25,7 +25,10 @@ pub use decoding::EventsDecodingError;
 pub use event_subscription::{
     subscribe,
     subscribe_finalized,
+    subscribe_to_block_headers_filling_in_gaps,
+    EventSub,
     EventSubscription,
+    FinalizedEventSub,
 };
 pub use events_type::{
     at,