Introduce OnReapAccount (#4585)

* Initial run and gun at `OnReapAccount`

* Fix some imports

* More fixes

* Whitespace

* More wack-a-mole

* Gotta catch em all

* Update lib.rs

* Small doc update

* Whitespace

substrate/frame/identity/src/lib.rs

@@ -921,6 +921,7 @@ mod tests {
 	impl pallet_balances::Trait for Test {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = ();
 		type TransferPayment = ();
@@ -950,6 +951,7 @@ mod tests {
 		type RegistrarOrigin = EnsureSignedBy<One, u64>;
 		type ForceOrigin = EnsureSignedBy<Two, u64>;
 	}
+	type System = frame_system::Module<Test>;
 	type Balances = pallet_balances::Module<Test>;
 	type Identity = Module<Test>;