[FRAME] Short-circuit fungible self transfer (#2118)

Changes:
- Change the fungible(s) logic to treat a self-transfer as No-OP (as
long as all pre-checks pass).

Note that the self-transfer case will not emit an event since no state
was changed.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

cumulus/primitives/utility/src/lib.rs

@@ -102,7 +102,7 @@ struct AssetTraderRefunder {
 /// Important: Errors if the Trader is being called twice by 2 BuyExecution instructions
 /// Alternatively we could just return payment in the aforementioned case
 pub struct TakeFirstAssetTrader<
-	AccountId,
+	AccountId: Eq,
 	FeeCharger: ChargeWeightInFungibles<AccountId, ConcreteAssets>,
 	Matcher: MatchesFungibles<ConcreteAssets::AssetId, ConcreteAssets::Balance>,
 	ConcreteAssets: fungibles::Mutate<AccountId> + fungibles::Balanced<AccountId>,
@@ -112,7 +112,7 @@ pub struct TakeFirstAssetTrader<
 	PhantomData<(AccountId, FeeCharger, Matcher, ConcreteAssets, HandleRefund)>,
 );
 impl<
-		AccountId,
+		AccountId: Eq,
 		FeeCharger: ChargeWeightInFungibles<AccountId, ConcreteAssets>,
 		Matcher: MatchesFungibles<ConcreteAssets::AssetId, ConcreteAssets::Balance>,
 		ConcreteAssets: fungibles::Mutate<AccountId> + fungibles::Balanced<AccountId>,
@@ -241,7 +241,7 @@ impl<
 }
 
 impl<
-		AccountId,
+		AccountId: Eq,
 		FeeCharger: ChargeWeightInFungibles<AccountId, ConcreteAssets>,
 		Matcher: MatchesFungibles<ConcreteAssets::AssetId, ConcreteAssets::Balance>,
 		ConcreteAssets: fungibles::Mutate<AccountId> + fungibles::Balanced<AccountId>,