Add slashing events to elections-phragmen. (paritytech#7543)

* Add slashing events to elections-phragmen.

* Fix build

* Apply suggestions from code review

* Update frame/elections-phragmen/src/lib.rs

* Update frame/elections-phragmen/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

frame/elections-phragmen/src/lib.rs

@@ -682,6 +682,10 @@ decl_event!(
 		/// A \[member\] has been removed. This should always be followed by either `NewTerm` or
 		/// `EmptyTerm`.
 		MemberKicked(AccountId),
+		/// A candidate was slashed due to failing to obtain a seat as member or runner-up
+		CandidateSlashed(AccountId, Balance),
+		/// A seat holder (member or runner-up) was slashed due to failing to retaining their position.
+		SeatHolderSlashed(AccountId, Balance),
 		/// A \[member\] has renounced their candidacy.
 		MemberRenounced(AccountId),
 		/// A voter was reported with the the report being successful or not.
@@ -995,13 +999,15 @@ impl<T: Trait> Module<T> {
 					new_runners_up_ids_sorted.binary_search(&c).is_err()
 				{
 					let (imbalance, _) = T::Currency::slash_reserved(&c, T::CandidacyBond::get());
+					Self::deposit_event(RawEvent::CandidateSlashed(c, T::CandidacyBond::get()));
 					T::LoserCandidate::on_unbalanced(imbalance);
 				}
 			});
 
 			// Burn outgoing bonds
 			to_burn_bond.into_iter().for_each(|x| {
 				let (imbalance, _) = T::Currency::slash_reserved(&x, T::CandidacyBond::get());
+				Self::deposit_event(RawEvent::SeatHolderSlashed(x, T::CandidacyBond::get()));
 				T::LoserCandidate::on_unbalanced(imbalance);
 			});