Add block-centric Storage API (#774)

* blocks: Add storage method

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Add support for runtime API calls and expose it to the blocks API

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* storage: Add storage type for block centric API

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Adjust subxt to the new Storage interface

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix clippy

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

subxt/src/blocks/block_types.rs

@@ -19,6 +19,7 @@ use crate::{
     events,
     rpc::types::ChainBlockResponse,
     runtime_api::RuntimeApi,
+    storage::Storage,
 };
 use derivative::Derivative;
 use futures::lock::Mutex as AsyncMutex;
@@ -91,6 +92,12 @@ where
         ))
     }
 
+    /// Work with storage.
+    pub fn storage(&self) -> Storage<T, C> {
+        let block_hash = self.hash();
+        Storage::new(self.client.clone(), block_hash)
+    }
+
     /// Execute a runtime API call at this block.
     pub async fn runtime_api(&self) -> Result<RuntimeApi<T, C>, Error> {
         Ok(RuntimeApi::new(self.client.clone(), self.hash()))