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/client/mod.rs

@@ -16,8 +16,5 @@ pub use online_client::{
     ClientRuntimeUpdater, OnlineClient, OnlineClientT, RuntimeUpdaterStream, Update, UpgradeError,
 };
 
-#[cfg(any(
-    feature = "jsonrpsee-ws",
-    all(feature = "jsonrpsee-web", target_arch = "wasm32")
-))]
+#[cfg(feature = "jsonrpsee")]
 pub use online_client::default_rpc_client;