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

testing/integration-tests/src/frame/system.rs

@@ -4,19 +4,21 @@
 
 use crate::{
     node_runtime::{self, system},
-    pair_signer, test_context,
+    test_context,
 };
 use assert_matches::assert_matches;
-use sp_keyring::AccountKeyring;
+use subxt_signer::sr25519::dev;
 
 #[tokio::test]
 async fn storage_account() -> Result<(), subxt::Error> {
     let ctx = test_context().await;
     let api = ctx.client();
 
-    let alice = pair_signer(AccountKeyring::Alice.pair());
+    let alice = dev::alice();
 
-    let account_info_addr = node_runtime::storage().system().account(alice.account_id());
+    let account_info_addr = node_runtime::storage()
+        .system()
+        .account(alice.public_key().to_account_id());
 
     let account_info = api
         .storage()
@@ -34,7 +36,7 @@ async fn tx_remark_with_event() -> Result<(), subxt::Error> {
     let ctx = test_context().await;
     let api = ctx.client();
 
-    let alice = pair_signer(AccountKeyring::Alice.pair());
+    let alice = dev::alice();
 
     let tx = node_runtime::tx()
         .system()