only invalidate referenced values

[Rich-Harris]

This PR means that changes to local state only trigger an invalidation if the state is referenced in the markup, or in a reactive declaration, meaning less work and less code:

function instance($$self, $$props, $$invalidate) {
  let seconds = 0;
	
  onMount(() => {
    const interval = setInterval(() => {
-      $$invalidate('seconds', seconds += 1);
+      seconds += 1;
      console.log(`the component has been mounted for ${seconds} seconds`);
    }, 1000);
		
    return () => {
      clearInterval(interval);
    };
  });

  return {};
}

A more practical example:

<script>
  import { setContext } from 'svelte';

  setContext('WEBGL_SCENE', {
    invalidate: () => {
      if (!update_scheduled) {
        update_scheduled = true;
        Promise.resolve().then(draw);
      }
    },

    // ...
  });

  function draw() {
    // this currently 'dirties' the component; i don't think it should
    update_scheduled = false;

    // ...draw implementation goes here...
  }
</script>

<slot></slot>

There is a small change to observable behaviour (other than avoiding unnecessary work) — beforeUpdate and afterUpdate will no longer fire when unreferenced local state changes. I think that's probably the correct behaviour anyway, since no 'update' is happening, and it's useful to be able to 'clear' those kinds of values in afterUpdate without triggering another update (which necessitates the infinite loop guard).

Thoughts?