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/lib.rs

@@ -133,6 +133,11 @@
 )]
 #![allow(clippy::type_complexity)]
 
+// Suppress an unused dependency warning because tokio is
+// only used in example code snippets at the time of writing.
+#[cfg(test)]
+use tokio as _;
+
 pub use subxt_macro::subxt;
 
 pub mod blocks;