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/examples/tx_basic.rs

@@ -1,5 +1,5 @@
-use sp_keyring::AccountKeyring;
-use subxt::{tx::PairSigner, OnlineClient, PolkadotConfig};
+use subxt::{OnlineClient, PolkadotConfig};
+use subxt_signer::sr25519::dev;
 
 // Generate an interface that we can use from the node's metadata.
 #[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata_small.scale")]
@@ -11,12 +11,12 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let api = OnlineClient::<PolkadotConfig>::new().await?;
 
     // Build a balance transfer extrinsic.
-    let dest = AccountKeyring::Bob.to_account_id().into();
+    let dest = dev::bob().public_key().into();
     let balance_transfer_tx = polkadot::tx().balances().transfer(dest, 10_000);
 
     // Submit the balance transfer extrinsic from Alice, and wait for it to be successful
     // and in a finalized block. We get back the extrinsic events if all is well.
-    let from = PairSigner::new(AccountKeyring::Alice.pair());
+    let from = dev::alice();
     let events = api
         .tx()
         .sign_and_submit_then_watch_default(&balance_transfer_tx, &from)