Fixes #299

This fixes stale values in computed stores by checking if dependencies have changed any time get() is called. It does have a perf impact, though I added an epoch optimization to help with that. And correctness should take precedence over performance (no other state library I'm aware of has this stale value problem). The benefits are:

• computed stores never return stale values
• this also solves the diamond problem, so the previous logic that was added for that is no longer needed, which means:
  • simpler logic
  • no need to store and compute a level (l) for each atom
  • bundle size reduced (min: 286B to 235B, max: 818B to 769B)
  • better perf in certain areas -- processing the listenerQueue is now O(n) instead of O(n^2)

Another potential benefit -- this change means that when calling get() on an atom with 0 subscribers, we no longer have to add a dummy subscriber. I'm holding off on making that change, because it would be a breaking change -- some code could be relying on the side effect of onMount firing. But it could be considered for a future major version -- IMO it would be cleaner. Right now, if you call get() on a computed store, it will never unsubscribe, and the value will be needlessly recomputed when the dependencies change, even if the only listener is the dummy listener.