Sort records for correct true/false positives

fixes #45 
What's heavyweight about it? It's probably fewer characters than the current comment, since it's not indented. All I'm suggesting is this:

```javascript
/**
 * Returns records which are in both a and b.
 *
 * NB: Expects a and b be sorted on recordKey, and unique on recordKey.
 *     Throws if unsorted (unless the unsorted portion of b is not reached)
 * time: O(n)
 */
function intersection(a, b) {
  ...
}
```