Rework `ViewDataDictionary` to ensure `ModelMetadata` is never `null` (#1466)

This is the reanimation of #1453, now done against dev.  No significant changes from that PR after I addressed comments there.

See individual commits for details.

/cc @pranavkm @rynowak @yishaigalatzer 
as discussed offline, I've paid the "documentation overhead" price already.  and what you suggest will both complicate the `ViewDataDictionary<TModel>` constructors (slightly) and duplicate logic between the two classes.