This could prevent it from updating values if you returned to
the original value. E.g., if your values went 1, 2, 1, 2, you'd
get notifications for 1, 2, 2. This would also cause some extra
notifications to go when the logical value didn't actually change.
This commit also includes an `observe2`, which operates directly on a
tuple without explicitly converting to an HList. I'm committing it for
posterity, but the next commit will remove it because it doesn't give a
narrow enough output type.
