Fix `Ord` instance for `Reward` type

Current `Ord` instance for `Reward` violates the law `compare r1 r2 == EQ ==> r1 == r2`

```haskell
instance Ord (Reward c) where
compare (Reward MemberReward _ _) (Reward LeaderReward _ _) = GT
compare (Reward LeaderReward _ _) (Reward MemberReward _ _) = LT
compare (Reward _ pool1 _) (Reward _ pool2 _) = compare pool1 pool2
```

This violates referential transparency and prevents libraries like `containers` that depend on `Ord` instance to function properly

In fact this bug is intentionally being abused in `aggregateRewards` and `filterReward` where `Set Reward` is being used.