Ensure heimdall is also aware of Cached Broccoli nodes

[stefanpenner]

• [x] adds heimdallNode.id.broccoliId:
  • lets us disambiguate broccoli nodes across rebuilds
  • lets us have multiple heimdall nodes pointing to a single broccoli node
• [x] adds heimdallNode.id.broccoliCachedNode
  • lets us disambiguate heimdall nodes which represent only already read broccoli nodes.
• [x] also instrument the existence of cached broccoli nodes

[stefanpenner]

likely next step: https://github.com/ember-cli/broccoli-viz/issues/36

[stefanpenner]

released as + broccoli-builder@0.18.3