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/wasm-tests/tests/wasm.rs

@@ -5,7 +5,13 @@ use wasm_bindgen_test::*;
 
 wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);
 
-/// Run the tests by `$ wasm-pack test --firefox --headless`
+// Run the tests by calling:
+//
+// ```text
+// wasm-pack test --firefox --headless`
+// ```
+//
+// You'll need to have a substrate/polkadot node running too with eg `substrate --dev`.
 
 #[wasm_bindgen_test]
 async fn wasm_ws_transport_works() {