New Event Subscription API (#442)

* Add reworked event types

* first pass implementing event subscriptions

* make clear that some methods are private

* comment tidy

* use Events in transaction stuff

* align transaction and event APIs

* remove __private_ prefixes; they are ugly

* fix examples and remove old events and subscription code

* better comments on hidden event functions

* re-add find_first_event; it's used a bunch in tests and examples

* cargo check --all-targets now passes

* Fix up existing event tests

* cargo fmt

* change todo to note

* clippy and doc niggles

* revert to find_first_event

* Add specific subscription related tests

* cargo fmt

* Update tests and add/fix examples

* cargo fmt

* add a little to subscribe_all_events example

* cargo fmt

* move an example comment

* easy access to root mod for more clarity

* add a couple of tests to ensure that events properly decoded until naff bytes

* Simplify EventSubscription Stream impl a little

* Address some PR feedback

examples/examples/balance_transfer.rs

@@ -0,0 +1,57 @@
+// Copyright 2019-2022 Parity Technologies (UK) Ltd.
+// This file is part of subxt.
+//
+// subxt is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// subxt is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+//! To run this example, a local polkadot node should be running. Example verified against polkadot 0.9.13-82616422d0-aarch64-macos.
+//!
+//! E.g.
+//! ```bash
+//! curl "https://github.com/paritytech/polkadot/releases/download/v0.9.13/polkadot" --output /usr/local/bin/polkadot --location
+//! polkadot --dev --tmp
+//! ```
+
+use sp_keyring::AccountKeyring;
+use subxt::{
+    ClientBuilder,
+    DefaultConfig,
+    DefaultExtra,
+    PairSigner,
+};
+
+#[subxt::subxt(runtime_metadata_path = "examples/polkadot_metadata.scale")]
+pub mod polkadot {}
+
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::init();
+
+    let signer = PairSigner::new(AccountKeyring::Alice.pair());
+    let dest = AccountKeyring::Bob.to_account_id().into();
+
+    let api = ClientBuilder::new()
+        .build()
+        .await?
+        .to_runtime_api::<polkadot::RuntimeApi<DefaultConfig, DefaultExtra<DefaultConfig>>>();
+    let hash = api
+        .tx()
+        .balances()
+        .transfer(dest, 10_000)
+        .sign_and_submit(&signer)
+        .await?;
+
+    println!("Balance transfer extrinsic submitted: {}", hash);
+
+    Ok(())
+}