Tighten up the semantics of `Ref`

We're very close now to a Ref having a precise meaning: A Referenced to a chunk valued in a particular database.

This is related to https://github.com/attic-labs/noms/issues/1573, but also somewhat orthogonal. The remaining bits of this are

1. Embed the ValueStore which has the referenced value in the Ref
2. Make TargetValue use the embedded ValueStore to retrieve the Value (and not take a ValueReader as an argument)
3. Remove the public constructors of Ref