Add tipping into treasury (#4480)

* First draft

* Initial work on tests

* Add tests.

* Ensure old members can't tip.

* Fix complexity

* Update node runtime

* Build fix.

* build fix

* Fix tests

* Fix tests

* Refactor Contains impl for tests

* Introduce new way to avoid impl Contains conflicts

* Fixes

* Docs.

* Docs.

* Typo

* Whitespace

* Docs

* Typo

* Formatting

* Update frame/treasury/src/lib.rs

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

* Update frame/treasury/src/lib.rs

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

* Update frame/treasury/src/lib.rs

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

* Apply suggestions from code review

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

* Add provisional weights.

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/elections-phragmen/src/lib.rs

@@ -88,7 +88,7 @@ use frame_support::{
 	decl_storage, decl_event, ensure, decl_module, decl_error, weights::SimpleDispatchInfo,
 	traits::{
 		Currency, Get, LockableCurrency, LockIdentifier, ReservableCurrency, WithdrawReasons,
-		ChangeMembers, OnUnbalanced, WithdrawReason
+		ChangeMembers, OnUnbalanced, WithdrawReason, Contains
 	}
 };
 use sp_phragmen::ExtendedBalance;
@@ -767,6 +767,13 @@ impl<T: Trait> Module<T> {
 	}
 }
 
+impl<T: Trait> Contains<T::AccountId> for Module<T> {
+	fn contains(who: &T::AccountId) -> bool {
+		Self::is_member(who)
+	}
+	fn sorted_members() -> Vec<T::AccountId> { Self::members_ids() }
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;