Wrap and export BlockNumber (#87)

* Add fetch example and wrap BlockNumber impl

* Fmt

examples/fetch_remote.rs

@@ -0,0 +1,45 @@
+// Copyright 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::System,
+    Error,
+    KusamaRuntime,
+};
+
+fn main() {
+    async_std::task::block_on(async move {
+        env_logger::init();
+
+        let block_hash = fetch_block_hash(1).await;
+        match block_hash {
+            Ok(Some(hash)) => println!("Block hash for block number 1: {}", hash),
+            Ok(None) => println!("Block number 1 not found."),
+            Err(_) => eprintln!("Failed to fetch block hash"),
+        }
+    });
+}
+
+async fn fetch_block_hash(
+    block_number: u32,
+) -> Result<Option<<KusamaRuntime as System>::Hash>, Error> {
+    substrate_subxt::ClientBuilder::<KusamaRuntime>::new()
+        .set_url("wss://kusama-rpc.polkadot.io")
+        .build()
+        .await?
+        .block_hash(Some(block_number.into()))
+        .await
+}