Add subxt_signer crate for native & WASM compatible signing (#1016)

* 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

subxt/src/tx/tx_client.rs

@@ -211,7 +211,7 @@ where
         Call: TxPayload,
         Signer: SignerT<T>,
     {
-        let account_nonce = self.account_nonce(signer.account_id()).await?;
+        let account_nonce = self.account_nonce(&signer.account_id()).await?;
         self.create_signed_with_nonce(call, signer, account_nonce, other_params)
     }