Extend the new api.blocks() to be the primary way to subscribe and fetch blocks/extrinsics/events (#691)

* First pass adding functions to get blocks and extrinsics

* cargo fmt and cache block events

* prefix block hash with 0x

* pin streams for better ergonomics and add an example of subscribing to blocks

* remove unused var

* standardise on _all, _best and _finalized for different block header subs

* WIP center subscribing around blocks

* Remove the event filtering/subscribing  stuff

* clippy

* we need tokio, silly clippy

* add extrinsic_index() call

* Update subxt/src/blocks/block_types.rs

Co-authored-by: Andrew Jones <ascjones@gmail.com>

Co-authored-by: Andrew Jones <ascjones@gmail.com>

subxt/src/blocks/mod.rs

@@ -4,8 +4,14 @@
 
 //! This module exposes the necessary functionality for working with events.
 
+mod block_types;
 mod blocks_client;
 
+pub use block_types::{
+    Block,
+    Extrinsic,
+    ExtrinsicEvents,
+};
 pub use blocks_client::{
     subscribe_to_block_headers_filling_in_gaps,
     BlocksClient,