Composite accounts (#4820)

* Basic account composition.

* Add try_mutate_exists

* De-duplicate

* Refactor away the UpdateBalanceOutcome

* Expunge final UpdateBalanceOutcome refs

* Refactor transfer

* Refactor reservable currency stuff.

* Test with the alternative setup.

* Fixes

* Test with both setups.

* Fixes

* Fix

* Fix macros

* Make indices opt-in

* Remove CreationFee, and make indices opt-in.

* Fix construct_runtime

* Fix last few bits

* Fix tests

* Update trait impls

* Don't hardcode the system event

* Make tests build and fix some stuff.

* Pointlessly bump runtime version

* Fix benchmark

* Another fix

* Whitespace

* Make indices module economically safe

* Migrations for indices.

* Fix

* Whilespace

* Trim defunct migrations

* Remove unused storage item

* More contains_key fixes

* Docs.

* Bump runtime

* Remove unneeded code

* Fix test

* Fix test

* Update frame/balances/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Fix ED logic

* Repatriate reserved logic

* Typo

* Fix typo

* Update frame/system/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/system/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Last few fixes

* Another fix

* Build fix

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Jaco Greeff <jacogr@gmail.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/staking/src/lib.rs

@@ -250,7 +250,6 @@
 mod mock;
 #[cfg(test)]
 mod tests;
-mod migration;
 mod slashing;
 
 pub mod inflation;
@@ -761,9 +760,6 @@ decl_storage! {
 
 		/// The earliest era for which we have a pending, unapplied slash.
 		EarliestUnappliedSlash: Option<EraIndex>;
-
-		/// The version of storage for upgrade.
-		StorageVersion: u32;
 	}
 	add_extra_genesis {
 		config(stakers):
@@ -795,8 +791,6 @@ decl_storage! {
 					}, _ => Ok(())
 				};
 			}
-
-			StorageVersion::put(migration::CURRENT_VERSION);
 		});
 	}
 }
@@ -1298,9 +1292,10 @@ impl<T: Trait> Module<T> {
 	}
 
 	/// Ensures storage is upgraded to most recent necessary state.
-	fn ensure_storage_upgraded() {
-		migration::perform_migrations::<T>();
-	}
+	///
+	/// Right now it's a no-op as all networks that are supported by Substrate Frame Core are
+	/// running with the latest staking storage scheme.
+	fn ensure_storage_upgraded() {}
 
 	/// Actually make a payment to a staker. This uses the currency's reward function
 	/// to pay the right payee for the given staker account.