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/src/book/setup/client.rs

@@ -18,23 +18,27 @@
 //! The provided clients are all generic over the [`crate::config::Config`] that they accept, which
 //! determines how they will interact with the chain.
 //!
-//! In the case of the [`crate::OnlineClient`], we have a few options to instantiate it:
+//! In the case of the [`crate::OnlineClient`], we have various ways to instantiate it:
 //!
-//! - [`crate::OnlineClient::new()`] to connect to a node running locally.
-//! - [`crate::OnlineClient::from_url()`] to connect to a node at a specific URL.
-//! - [`crate::OnlineClient::from_rpc_client()`] to instantiate the client with a custom RPC
-//!   implementation.
+//! - [`crate::OnlineClient::new()`] to connect to a node running locally. This uses the default Subxt
+//!   backend, and the default RPC client.
+//! - [`crate::OnlineClient::from_url()`] to connect to a node at a specific URL. This uses the default Subxt
+//!   backend, and the default RPC client.
+//! - [`crate::OnlineClient::from_rpc_client()`] to instantiate the client with a [`crate::backend::rpc::RpcClient`].
+//! - [`crate::OnlineClient::from_backend()`] to instantiate Subxt using a custom backend. Currently there
+//!   is just one backend, [`crate::backend::legacy::LegacyBackend`]. This backend can be instantiated from
+//!   a [`crate::backend::rpc::RpcClient`].
 //!
-//! The latter accepts anything that implements the low level [`crate::rpc::RpcClientT`] trait; this
-//! allows you to decide how Subxt will attempt to talk to a node if you'd prefer something other
-//! than the provided interfaces. Under the hood, this is also how the light client is implemented.
+//! [`crate::backend::rpc::RpcClient`] can itself be instantiated from anything that implements the low level
+//! [`crate::backend::rpc::RpcClientT`] trait; this allows you to decide how Subxt will attempt to talk to a node
+//! if you'd prefer something other default client. We use this approach under the hood to implement the light client.
 //!
 //! ## Examples
 //!
 //! Most of the other examples will instantiate a client. Here are a couple of examples for less common
 //! cases.
 //!
-//! ### Writing a custom [`crate::rpc::RpcClientT`] implementation:
+//! ### Writing a custom [`crate::backend::rpc::RpcClientT`] implementation:
 //!
 //! ```rust,ignore
 #![doc = include_str!("../../../examples/setup_client_custom_rpc.rs")]