This adds an overload of JSGraph::Constant that takes an ObjectReference
rather than a Handle<Object>.

ObjectReference is a new superclass of HeapReference.

Also several refactorings and renaming, e.g.:
- Rename HeapReference to HeapObjectRef.
- Rename ContextHeapReference to ContextRef.
- ...
- Rename HeapReferenceType to HeapObjectType.

Bug: v8:7790
Change-Id: Id3e567cbaf7c326189b99b2fd4ced6bff02f9640
Reviewed-on: https://chromium-review.googlesource.com/1104337Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53797}