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

signer/wasm-tests/tests/wasm.rs

@@ -0,0 +1,26 @@
+#![cfg(target_arch = "wasm32")]
+
+use subxt_signer::sr25519;
+use wasm_bindgen_test::*;
+
+wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);
+
+// Run the tests by calling:
+//
+// ```text
+// wasm-pack test --firefox --headless`
+// ```
+//
+// These are independent of any other package to ensure that nothing
+// else enabled the same feature flag that subxt-signer needs to work ok
+// (subxt seems to, for instance).
+
+#[wasm_bindgen_test]
+async fn wasm_signing_works() {
+    let alice = sr25519::dev::alice();
+
+    // There's some non-determinism in the signing, so this ensures that
+    // the rand stuff is configured properly to run ok in wasm.
+    let signature = alice.sign(b"Hello there");
+    assert!(sr25519::verify(&signature, b"Hello there", &alice.public_key()));
+}