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/error.rs

@@ -63,6 +63,9 @@ pub enum Error {
     /// Transaction progress error.
     #[error("Transaction error: {0}")]
     Transaction(#[from] TransactionError),
+    /// Block related error.
+    #[error("Block error: {0}")]
+    Block(#[from] BlockError),
     /// An error encoding a storage address.
     #[error("Error encoding storage address: {0}")]
     StorageAddress(#[from] StorageAddressError),
@@ -237,6 +240,24 @@ impl DispatchError {
     }
 }
 
+/// Block error
+#[derive(Clone, Debug, Eq, thiserror::Error, PartialEq)]
+pub enum BlockError {
+    /// The block
+    #[error(
+        "Could not find a block with hash {0} (perhaps it was on a non-finalized fork?)"
+    )]
+    BlockHashNotFound(String),
+}
+
+impl BlockError {
+    /// Produce an error that a block with the given hash cannot be found.
+    pub fn block_hash_not_found(hash: impl AsRef<[u8]>) -> BlockError {
+        let hash = format!("0x{}", hex::encode(hash));
+        BlockError::BlockHashNotFound(hash)
+    }
+}
+
 /// Transaction error.
 #[derive(Clone, Debug, Eq, thiserror::Error, PartialEq)]
 pub enum TransactionError {