fromUniqDynamic should be in MonadHold

[ryantrinkle]

Just like holdUniqDynamic, when a type has an Eq instance such that a == b does not guarantee that forall f. f a == f b, the time at which deduplication starts becomes significant. Since we have no way of determining whether the user's Eq instance has this property or not, we should make them specify the time by using MonadHold.