Ref Values now have a TargetRef() method that returns the ref.Ref of
the target the Value is referencing.
Note: This is a breaking change. In old code the Ref() of the Value was
the Ref of the underlying target.
Fixes#464
This makes the new typed serialization the default (the old
serialization is not used but the code has not been cleaned up yet).
Some things are no working in the new world:
Chunking - The compound list is not working correctly any more. The
Chunks method is having issues because it assumed things based on the
old implicit chunking.
Commit - uses a `Set(Commit)` which means that the parent commit is
embedded. We need to change that to be `Set(Ref(Commit))` so that the
parent commit is referenced instead.
This will enable us to walk the chunk graph without having to go
through weird contortions to figure out which values don't have
chunks in any chunkstore (because they were inlined).
Towards issue #82
Erik Arvidsson