In pursuit of issue #654, we want to be able to figure out all the
refs contained in a given Value, along with the Types of the Values to
which those refs point. Value.Chunks() _almost_ met those needs, but
it returned a slice of ref.Ref, which doesn't convey any type info.
To address this, this patch does two things:
1) RefBase embeds the Value interface, and
2) Chunks() now returns []types.RefBase
RefBase now provides Type() as well, by virtue of embedding Value, so
callers can just iterate through the slice returned from Chunks() and
gather type info for all the refs embedded in a given Value.
I went all the way and made RefBase a Value instead of just adding the
Type() method because both types.Ref and the generated Ref types are
actually all Values, and doing so allowed me to change the definition of
refBuilderFunc in package_registry.go to be more precise. It now returns
RefBase instead of just Value.
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
