* 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
* Add and use subxt-signer crate for WASM compatible signing
* cargo fmt
* dev keypairs already references
* WIP fix various breakages
* re-jig features to be simpler and various test fixes etc
* doc and web fix
* fix various bits and pieces
* fix a test I broke
* dev-deps can't be linked to in docs, hrmph
* cargo fmt
* another doc link
* document the subxt_signer crate more thoroughly
* move feature flag for consistency
* more docs, no default subxt feature flag on signer, update release instrs
* Add missing license header
* unwrap_inner => into_inner
* extend a test a little to better check derive junctions
* note more clearly that the crypto bits come from sp_core::crypto
James Wilson