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

cli/src/commands/explore/calls.rs

@@ -125,10 +125,9 @@ fn mocked_offline_client(metadata: Metadata) -> OfflineClient<SubstrateConfig> {
         H256::from_str("91b171bb158e2d3848fa23a9f1c25182fb8e20313b2c1eb49219da7a70ce90c3")
             .expect("Valid hash; qed");
 
-    let runtime_version = subxt::rpc::types::RuntimeVersion {
+    let runtime_version = subxt::backend::RuntimeVersion {
         spec_version: 9370,
         transaction_version: 20,
-        other: Default::default(),
     };
 
     OfflineClient::<SubstrateConfig>::new(genesis_hash, runtime_version, metadata)