Make storage futures only borrow client, not self, for better ergonomics (#561)

* Allow storage requests to be made concurrently

* fmt and license

* Update codegen/src/api/storage.rs

Capitalise comment

Co-authored-by: Tarik Gul <47201679+TarikGul@users.noreply.github.com>

* Update codegen/src/api/storage.rs

Capitalise comment

Co-authored-by: Tarik Gul <47201679+TarikGul@users.noreply.github.com>

* split should_ref things up to move them closer to where they are used

* fmt + clippy

Co-authored-by: Tarik Gul <47201679+TarikGul@users.noreply.github.com>

examples/examples/concurrent_storage_requests.rs

@@ -0,0 +1,46 @@
+// 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/>.
+
+use futures::join;
+use sp_keyring::AccountKeyring;
+use subxt::{
+    ClientBuilder,
+    DefaultConfig,
+    PolkadotExtrinsicParams,
+};
+
+#[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata.scale")]
+pub mod polkadot {}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let api = ClientBuilder::new()
+        .build()
+        .await?
+        .to_runtime_api::<polkadot::RuntimeApi<DefaultConfig, PolkadotExtrinsicParams<DefaultConfig>>>();
+
+    let addr = AccountKeyring::Bob.to_account_id();
+
+    // For storage requests, we can join futures together to
+    // await multiple futures concurrently:
+    let a_fut = api.storage().staking().bonded(&addr, None);
+    let b_fut = api.storage().staking().ledger(&addr, None);
+    let (a, b) = join!(a_fut, b_fut);
+
+    println!("{a:?}, {b:?}");
+
+    Ok(())
+}