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 <[email protected]>

* Fix ED logic

* Repatriate reserved logic

* Typo

* Fix typo

* Update frame/system/src/lib.rs

Co-Authored-By: Shawn Tabrizi <[email protected]>

* Update frame/system/src/lib.rs

Co-Authored-By: Shawn Tabrizi <[email protected]>

* Last few fixes

* Another fix

* Build fix

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Jaco Greeff <[email protected]>
Co-authored-by: Shawn Tabrizi <[email protected]>