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/bin/node/runtime/src/lib.rs

@@ -174,6 +174,7 @@ parameter_types! {
 impl pallet_balances::Trait for Runtime {
 	type Balance = Balance;
 	type OnFreeBalanceZero = ((Staking, Contracts), Session);
+	type OnReapAccount = System;
 	type OnNewAccount = Indices;
 	type Event = Event;
 	type DustRemoval = ();