add simple tracing API

The API works as:

```javascript
  System.trace = true;

  System.import('some/module').then(function() {
    // System.deps['some/module'] == ['direct', 'normalized', 'deps'];
    // System.deps['normalized'] == [];
    // System.deps['deps'] == ['normalized'];
    // System.deps['direct'] == [];
  });
```

If this seems obscure, let me know - happy to consider other APIs too.