add simple tracing API

The API works as:

```javascript
  System.trace = true;
  System.execute = false; // optionally disable execution. Still executes wrappers.

  System.import('some/module').then(function() {
    // System.loads['some/module'] is the load record
    // System.loads['some/module'].dependencies == [{ name, value from unnormalized to normalized dependencies }]
    // System.loads['some/module'].metadata is metadata object
    // System.loads['some/module'].address is the resolved address
    // System.loads['some/module'].source is the translated source string
  });
```

The information above provides everything a build tool would need - translation, resolution, dependencies.

If this seems obscure, let me know - happy to consider other APIs too.