Make signing fallable and asynchronous

This is needed for hardware wallets, which require human confirmation to
sign transactions.  Blocking on a human to sign transactions is not a
good idea, and the signing might fail for many reasons (device
unplugged, authorization not granted, etc).

src/frame/balances.rs

@@ -16,21 +16,11 @@
 
 //! Implements support for the pallet_balances module.
 
-use crate::frame::system::{
-    System,
-    SystemEventsDecoder,
-};
-use codec::{
-    Decode,
-    Encode,
-};
+use crate::frame::system::{System, SystemEventsDecoder};
+use codec::{Decode, Encode};
 use core::marker::PhantomData;
 use frame_support::Parameter;
-use sp_runtime::traits::{
-    AtLeast32Bit,
-    MaybeSerialize,
-    Member,
-};
+use sp_runtime::traits::{AtLeast32Bit, MaybeSerialize, Member};
 use std::fmt::Debug;
 
 /// The subset of the `pallet_balances::Trait` that a client must implement.
@@ -110,10 +100,7 @@ pub struct TransferEvent<T: Balances> {
 mod tests {
     use super::*;
     use crate::{
-        system::{
-            AccountStore,
-            AccountStoreExt,
-        },
+        system::{AccountStore, AccountStoreExt},
         tests::test_client,
     };
     use sp_keyring::AccountKeyring;