fix: update indirectly-affected bindings on mutation

[Rich-Harris]

WIP alternative to #16165. The real fix, I think, is to not use effects for synchronization at all, but rather to invalidate indirect bindings on mutation. In other words in a case like this...

<script>
  export let selected;
  export let tasks;
</script>

<select bind:value={selected}>
  {#each tasks as task}
    <option value='{task}'>{task.description}</option>
  {/each}
</select>

<label>
  <input type='checkbox' bind:checked={selected.done}> {selected.description}
</label>

<h2>Pending tasks</h2>
{#each tasks.filter(t => !t.done) as task}
  <p>{task.description}</p>
{/each}

...updating selected should also update tasks, because the bindings are linked. I think we might be able to use this mechanism for each blocks as well and end up with simpler compiler code, though I don't have time to finish it right now and my brain needs a rest anyway because this stuff is confusing as hell. Can't wait to be able to delete all this legacy gubbins.

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