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/tx/mod.rs

@@ -52,7 +52,6 @@ pub use self::{
         TxPayload,
     },
     tx_progress::{
-        TxEvents,
         TxInBlock,
         TxProgress,
         TxStatus,