Add count to array computed meta

As discussed with @hjdivad and @mmun this adds the change count to `changeMeta`. I've extracted this from work on `Ember.computed.slice` which is waiting for some feedback before I can PR it. This can probably help with performance on `Ember.computed.sort` out of the box.

```
Ember.computed.sort = function(itemsKey, begin, end) {

  var changsInBatch = 0;

  return Ember.arrayComputed(itemsKey, {
    /* ... */
    removedItem: function (array, item, changeMeta, instanceMeta) {
      if (changesInBatch > 1) {
        changesInBatch--;
      } else {
        changesInBatch = changeMeta.changeCount;
      }
      // changesInBatch is how many operations remain (including the current one)
    }
  });
};
```

Maybe this work should just go straight into  a patch for `sort`? I wanted to get it out there regardless.