Glitch free reactive stores

[Rich-Harris]

Started replying to @marvinhagemeister on sveltejs/rfcs#5 but got side-tracked by trying to see if we could make our reactive stores glitch-free (i.e. if a derived store depends on a and b, and b is also a derived store that depends on a, don't recalculate when a changes until b is also up-to-date).

I think this works? But would appreciate a sanity check if anyone is familiar with this problem space...

Explanation: each store's subscribe method now takes an optional second invalidate method:

store.subscribe(
  value => console.log(`the value is`, value),
  () => console.log(`here comes a new value... I wonder what it will be?`)
);

When a store's value changes, it should call all the invalidate functions and then call the main callback. Most of the time the second argument is unnecessary, but the derive function uses it to update an internal number that keeps track of which inputs are invalid.