* 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
James Wilson