Storage map iter (#148)

* Add example file

* Fmt

* Add KeyIter.

* Add iter method to store proc-macro.

* Fetch all values at once.

* Update docs.

* Run rustfmt.

Co-authored-by: Andrew Jones <ascjones@gmail.com>

examples/fetch_all_accounts.rs

@@ -0,0 +1,33 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of substrate-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 substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+use substrate_subxt::{
+    system::AccountStoreExt,
+    ClientBuilder,
+    DefaultNodeRuntime,
+};
+
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::init();
+
+    let client = ClientBuilder::<DefaultNodeRuntime>::new().build().await?;
+    let mut iter = client.account_iter(None).await?;
+    while let Some((key, account)) = iter.next().await? {
+        println!("{:?}: {}", key, account.data.free);
+    }
+    Ok(())
+}