Introduce Backend trait to allow different RPC (or other) backends to be implemented (#1126)

* WIP backend trait

* WIP converting higher level stuff to using Backend impl

* more implementing new backend trait, mainly storage focused

* Get core code compiling with new backend bits

* subxt crate checks passing

* fix tests

* cargo fmt

* clippy/fixes

* merging and other fixes

* fix test

* fix lightclient code

* Fix some broken doc links

* another book link fix

* fix broken test when moving default_rpc_client

* fix dry_run test

* fix more tests; lightclient and wasm

* fix wasm tests

* fix some doc examples

* use next() instead of next_item()

* missing next_item() -> next()s

* move legacy RPc methods to LegacyRpcMethods type to host generic param instead of RpcClient

* standardise on all RpcClient types prefixed with Rpc, and 'raw' trait types prefixed with RawRpc so it's less ocnfusing which is which

* rename fixes

* doc fixes

* Add back system_dryRun RPC method and rename tx.dry_run() to tx.validate(), to signal that the calls are different

* Add a test that we return the correct extrinsic hash from submit()

* add TransactionValid details back, and protect against out of range bytes

* add test for decoding transaction validation from empty bytes

* fix clippy warning

subxt/examples/blocks_subscribing.rs

@@ -1,4 +1,3 @@
-use futures::StreamExt;
 use subxt::{OnlineClient, PolkadotConfig};
 
 #[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata_small.scale")]
@@ -24,8 +23,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         println!("  Extrinsics:");
 
         // Log each of the extrinsic with it's associated events:
-        let body = block.body().await?;
-        for ext in body.extrinsics().iter() {
+        let extrinsics = block.extrinsics().await?;
+        for ext in extrinsics.iter() {
             let ext = ext?;
             let idx = ext.index();
             let events = ext.events().await?;