Use tokio instead of async-std (#495)

* examples: Use tokio instead of std async

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* test-runtime: Use tokio instead of std async

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* subxt: Use tokio instead of std async

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* examples: Use only necessary tokio features

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

examples/examples/balance_transfer.rs

@@ -33,7 +33,7 @@ use subxt::{
 #[subxt::subxt(runtime_metadata_path = "examples/polkadot_metadata.scale")]
 pub mod polkadot {}
 
-#[async_std::main]
+#[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     env_logger::init();